Add a dependency to libgpg-error

I just tried to update `gmime` and `glib` (see #18830 and #19222). The build failed with the following error (relevant bit only):

    test-pkcs7.c:36:10: fatal error: 'gpg-error.h' file not found
    #include <gpg-error.h>
             ^
    1 error generated.
    make[2]: *** [test-pkcs7.o] Error 1

So, taking a guess, I installed `libgpg-error` and tried again. It worked. So I take it that `gmime` should depend on `libgpg-error` now. (After talking with @mistydemeo in irc, I tested and it seems to be a `:build`-only dependency.)