run gpgme_check_version() in JNI_OnLoad to init thread support

"The function gpgme_check_version must be called before any other function
in the library, because it initializes the thread support subsystem in
GPGME. To achieve this in multi-threaded programs, you must synchronize the
memory with respect to other threads that also want to use GPGME. For this,
it is sufficient to call gpgme_check_version before creating the other
threads using GPGME"
https://www.gnupg.org/documentation/manuals/gpgme/Multi-Threading.html