Improve C++ compatibility of previous patch.
* src/gpgme.h.in: Move gpgme_sssize_t and gpgme_off_t typedefs into the extern "C" scope.
Frankly, this does not help very much because g++ still does not
interpret gpgme_data_seek_cb_t as compatible with the C++ ssize_t. I
am sorry for that API change but C and C++ are too different to always
get both of them under one umbrella. However, the ABI should now be
identical to older gpgme versions.