[gimp] app: fixes int overflow warning.

[Jehan <jehanp src gnome org>]

commit bb7f229b97e5ef8dd5f5c0b64bfebfdf4867de02
Author: Jehan <jehan girinstud io>
Date:   Wed Dec 25 22:01:33 2019 +0100

    app: fixes int overflow warning.
    
    This fixes the following warning when compiling with Mingw:
    > warning: integer overflow in expression of type 'long int' results in '-694967296' [-Woverflow]

 app/gimp-update.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)
---
diff --git a/app/gimp-update.c b/app/gimp-update.c
index 4789a5a302..68d8d56b59 100644
--- a/app/gimp-update.c
+++ b/app/gimp-update.c
@@ -178,7 +178,7 @@ gimp_update_check (GimpCoreConfig *config)
   current_timestamp = g_get_real_time();
 
   /* Do not check more than once a week. */
-  if (current_timestamp < prev_update_timestamp + G_USEC_PER_SEC * 3600L * 24L * 7L)
+  if (current_timestamp - prev_update_timestamp < (gint64) G_USEC_PER_SEC * 3600L * 24L * 7L)
     return FALSE;
 
   gimp_versions = g_file_new_for_uri ("https://testing.gimp.org/gimp_versions.json";);