JCE: add synchronization to protected methods in WolfCryptSignature

[cconlon]

This PR adds synchronization to protected methods in WolfCryptSignature class.

This fixes a sporadic JUnit failure that looks similar to:

Threading error in RSA sign/verify thread test
junit.framework.AssertionFailedError: Threading error in RSA sign/verify thread test
	at com.wolfssl.provider.jce.test.WolfCryptSignatureTest.threadRunnerSignVerify(WolfCryptSignatureTest.java:448)
	at com.wolfssl.provider.jce.test.WolfCryptSignatureTest.testThreadedWolfSignWolfVerify(WolfCryptSignatureTest.java:462)
	at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
	at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)