build: Minor API fixes to fix build problems on AIX.

* src/gcrypt.h.in (gcry_error_from_errno): Fix return type.
* src/visibility.c (gcry_md_extract): Change return type to match the
prototype.

IBM compiler optimize enums and thus enums may be shorter than an
unsigned int. Thus an

assert (sizeof (gpg_error_t) == sizeof (gpg_err_code_t)

would fail. The deatils seem to depend on the passed compiler options
which explains that it has been only reported now.

• GnuPG-bug-id: T3256
• Signed-off-by: Werner Koch <wk@gnupg.org>