fix crash when encrypting a file with no keys in keyring

refs #3061 https://dev.guardianproject.info/issues/3061
replace hack using private header with public gpgme_signers_count()

This is how I originally figured out how to get it working, then Werner
added the gpgme_signers_count() function as of gpgme 1.4.3.

fixes #1549 https://dev.guardianproject.info/issues/1549