[gimp] meson: add -Wformat to common cflags.

[Jehan <jehanp src gnome org>]

commit fdf5c7edce35456e44ec776baf58add0459c6dc2
Author: Jehan <jehan girinstud io>
Date:   Wed Nov 6 12:54:04 2019 +0100

    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 | 1 +
 1 file changed, 1 insertion(+)
---
diff --git a/meson.build b/meson.build
index 2eb6f17d77..6249e5fa79 100644
--- a/meson.build
+++ b/meson.build
@@ -231,6 +231,7 @@ warning_cflags_common = [
   '-fdiagnostics-show-option',
   '-fno-common',
 
+  '-Wformat',
   '-Wformat-security',
   '-Winit-self',
   '-Wlogical-op',