w32: Explicitly link ws2_32

* src/Makefile.am (gpgme_w32_extra_libs): New.
(libgpgme_la_LIBADD, libgpgme_glib_la_LIBADD): Use it.

Since w32-io.c directly uses functions from ws2_32 it should
not rely on libgpg-error to pull in this dependency.