FindJava.cmake: ignore javah for JDK 10, it has been removed

The standalone javah tool has been removed, header generation happens during execution of javac. "javac -h" allows generation of headers only.

See http://openjdk.java.net/projects/jdk/10/ and http://openjdk.java.net/jeps/313 for details.

Sufficient to build VTK with JDK 10, other toolchains/projects may need further changes.

Fixes: #17613 (closed)
Topic-rename: FindJava-jdk-10