meson: add -Wformat to common cflags.

This fixes the following warning during GIR generation:
> cc1: warning: -Wformat-security ignored without -Wformat [-Wformat-security]

And anyway it is useless to set -Wformat-security if -Wformat is not set
as well, so if we want one, let's have both.

meson.build

@@ -231,6 +231,7 @@ warning_cflags_common = [
   '-fdiagnostics-show-option',
   '-fno-common',
 
+  '-Wformat',
   '-Wformat-security',
   '-Winit-self',
   '-Wlogical-op',