Fix build to work with Java 9

[INFO] Running org.wildfly.security.x500.X500AttributePrincipalDecoderTest
[ERROR] Tests run: 4, Failures: 0, Errors: 4, Skipped: 0, Time elapsed: 0.019 s <<< FAILURE! - in org.wildfly.security.x500.X500AttributePrincipalDecoderTest
[ERROR] testDecodeWithRequiredAttributes(org.wildfly.security.x500.X500AttributePrincipalDecoderTest)  Time elapsed: 0.013 s  <<< ERROR!
java.lang.IllegalAccessError: access to public member failed: sun.security.x509.X500Name.asX500Principal()X500Principal/invokeVirtual, from public Lookup
	at java.base/java.lang.invoke.MemberName.makeAccessException(MemberName.java:914)
	at java.base/java.lang.invoke.MethodHandles$Lookup.checkAccess(MethodHandles.java:2193)
	at java.base/java.lang.invoke.MethodHandles$Lookup.checkMethod(MethodHandles.java:2133)
	at java.base/java.lang.invoke.MethodHandles$Lookup.getDirectMethodCommon(MethodHandles.java:2282)
	at java.base/java.lang.invoke.MethodHandles$Lookup.getDirectMethodNoSecurityManager(MethodHandles.java:2276)
	at java.base/java.lang.invoke.MethodHandles$Lookup.unreflect(MethodHandles.java:1749)
	at org.wildfly.security.x500.X500PrincipalUtil.<clinit>(X500PrincipalUtil.java:54)
	at org.wildfly.security.x500.X500AttributePrincipalDecoder.getName(X500AttributePrincipalDecoder.java:177)
	at org.wildfly.security.x500.X500AttributePrincipalDecoderTest.testDecodeWithRequiredAttributes(X500AttributePrincipalDecoderTest.java:89)

[ERROR] testDecodeWithConcatenation(org.wildfly.security.x500.X500AttributePrincipalDecoderTest)  Time elapsed: 0.001 s  <<< ERROR!
java.lang.NoClassDefFoundError: Could not initialize class org.wildfly.security.x500.X500PrincipalUtil
	at org.wildfly.security.x500.X500AttributePrincipalDecoder.getName(X500AttributePrincipalDecoder.java:177)
	at org.wildfly.security.auth.server.PrincipalDecoder.lambda$concatenating$3(PrincipalDecoder.java:111)
	at org.wildfly.security.x500.X500AttributePrincipalDecoderTest.testDecodeWithConcatenation(X500AttributePrincipalDecoderTest.java:70)

[ERROR] testDecodeAttributeWithSubrange(org.wildfly.security.x500.X500AttributePrincipalDecoderTest)  Time elapsed: 0.001 s  <<< ERROR!
java.lang.NoClassDefFoundError: Could not initialize class org.wildfly.security.x500.X500PrincipalUtil
	at org.wildfly.security.x500.X500AttributePrincipalDecoder.getName(X500AttributePrincipalDecoder.java:177)
	at org.wildfly.security.x500.X500AttributePrincipalDecoderTest.testDecodeAttributeWithSubrange(X500AttributePrincipalDecoderTest.java:52)

[ERROR] testDecodeInReverse(org.wildfly.security.x500.X500AttributePrincipalDecoderTest)  Time elapsed: 0 s  <<< ERROR!
java.lang.NoClassDefFoundError: Could not initialize class org.wildfly.security.x500.X500PrincipalUtil
	at org.wildfly.security.x500.X500AttributePrincipalDecoder.getName(X500AttributePrincipalDecoder.java:177)
	at org.wildfly.security.x500.X500AttributePrincipalDecoderTest.testDecodeInReverse(X500AttributePrincipalDecoderTest.java:40)