notmuch: patch source to use full path of gpg2

gets rid of the dependency on the old gnupg1. and enables decryption
even if the gpg binary is not in the users environment.

warning: i don't know whether the switch from gnupg1 to gnupg
introduces any incompatibilities. this is a works for me patch.
Merge pull request #482 from coroa/notmuch-gpg2

notmuch: patch source to use full path of gpg2