gimp: fix build on darwin

despite us currently having a gtk2 without `GDK_OSX_BIGSUR` defined,
gimp have gone and spelt it `GDK_OSX_BIG_SUR` so define this
manually anyway

pkgs/applications/graphics/gimp/default.nix

···
   # test-eevl.c:64:36: error: initializer element is not a compile-time constant
   doCheck = !stdenv.isDarwin;
 
+  NIX_CFLAGS_COMPILE = lib.optional stdenv.isDarwin "-DGDK_OSX_BIG_SUR=16";
+
   # Check if librsvg was built with --disable-pixbuf-loader.
   PKG_CONFIG_GDK_PIXBUF_2_0_GDK_PIXBUF_MODULEDIR = "${librsvg}/${gdk-pixbuf.moduleDir}";