glib: Work around AppleClang + -Werror test build failure
Hi,
This MR is to work around an error I found with Xcode 13.2.1 when building glib. The error comes from using function prototypes that are conditionally defined, and this one in particular isn't included in the test preamble. That is not allowed, and is enforced by the combination of -std=gnu99 and -Werror.