tests: fix java JNI test assert for jni.h existing

compiler.has_header() isn't an assert, even though the comments say it
is. With `required: true` it is an actual assert.

test cases/java/9 jni/meson.build

@@ -17,9 +17,9 @@ java = find_program('java')
 jni_dep = dependency('jni', version : '>=1.8', modules: ['jvm', 'awt'])
 
 # Assert that the header can actually be found with the dependency.
-cc.has_header('jni.h', dependencies: [jni_dep])
+cc.has_header('jni.h', dependencies: [jni_dep], required: true)
 # Assert that the platform-specific include directory is included in the compiler arguments.
-cc.has_header('jni_md.h', dependencies: [jni_dep])
+cc.has_header('jni_md.h', dependencies: [jni_dep], required: true)
 
 # generate native headers
 subdir('src')