core: Make sure to stay ABI compatible.

* src/gpgme.h.in (struct _gpgme_key_sig): Move trust_scope to the end
of the struct.

Note that we assume that splitting a reserved 28 bit field into 3
fields does not change the ABI.

• Fixes-commit: 276187f6b62a73e92b1c20f589510f80ab9e88d2