meson: Add missing argument to gssapi.h check
These were missing since the initial introduction of the meson based build, in
e6927270cd18. As-is this is unlikely to cause an issue, but a future commit
will add support for detecting gssapi without use of dependency(), which could
fail due to this.
Discussion: https://postgr.es/m/
20240708225659.gmyqoosi7km6ysgn@awork3.anarazel.de
Backpatch: 16-, where the meson based build was added