cephfs-java: build against older jni headers

Older versions of the JNI interface expected non-const parameters
to their memory move functions. It's unpleasant, but won't actually
change the memory in question, to do a cast_const in order to satisfy
those older headers. (And even if it *did* modify the memory, that
would be okay given our single user.)

Signed-off-by: Greg Farnum <greg@inktank.com>
Merge pull request #1854 from ceph/wip-java-build

cephfs-java: build against older jni headers

Reviewed-by: Samuel Just <sam.just@inktank.com>