meson: make pointer-sign warning critical

With the previous commit, no pointer signedness warning should be
triggered even built with C23. Let's make the warning critical.

meson.build

@@ -391,6 +391,7 @@ possible_common_cc_flags = [
         '-Werror=missing-prototypes',
         '-Werror=overflow',
         '-Werror=override-init',
+        '-Werror=pointer-sign',
         '-Werror=return-type',
         '-Werror=shift-count-overflow',
         '-Werror=shift-overflow=2',