shim: Ensure we disable GTK_MODULES (thanks @cybre!)

Same reason as XMODIFIERS - it'll stop Steam starting up properly

Signed-off-by: Ikey Doherty <ikey@solus-project.com>

src/shim/shim.c

@@ -76,6 +76,7 @@ int main(int argc, char **argv)
          * properly use D-BUS ...
          */
         unsetenv("XMODIFIERS");
+        unsetenv("GTK_MODULES");
 
         memset(&n_argv, 0, sizeof(char *) * (argc + 2));