libgpgme: fix musl build with argp-standalone

The libgpgme configure script does not detect the error_t type under musl,
because musl does not define error_t in errno.h. As a result config.h adds a
local definition of error_t. When argp-standalone is present this local
definition collides with the error_t typedef in argp.h. Assume we always have
error_t when argp-standalone is present.

Fixes:
http://autobuild.buildroot.net/results/656/6566a1c77983404d90920997bfd15bd98c7ddde7/
http://autobuild.buildroot.net/results/a7b/a7b92ad68ff6e13d20ff44e955dc5748b2efd6a4/
http://autobuild.buildroot.net/results/0db/0db1b2aa0969d6cbae57b1400b7b766a5988abf9/



Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>