[gimp] app: fix "_WIN32_WINNT" redefined when cross-compiling with mingw-w64.

[Jehan Pagès <jehanp src gnome org>]

commit 2bb0f1b1c97bf350aa3034faf41b7d9dba5c6ec2
Author: Jehan <jehan girinstud io>
Date:   Sun May 29 02:26:04 2016 +0200

    app: fix "_WIN32_WINNT" redefined when cross-compiling with mingw-w64.
    
    It was already defined in _mingw.h, provided with mingw-w64 headers.

 app/main.c |    3 +++
 1 files changed, 3 insertions(+), 0 deletions(-)
---
diff --git a/app/main.c b/app/main.c
index 08f1475..ab68d95 100644
--- a/app/main.c
+++ b/app/main.c
@@ -75,6 +75,9 @@
 /* To get PROCESS_DEP_* defined we need _WIN32_WINNT at 0x0601. We still
  * use the API optionally only if present, though.
  */
+#ifdef _WIN32_WINNT
+#undef _WIN32_WINNT
+#endif
 #define _WIN32_WINNT 0x0601
 #include <windows.h>
 #include <conio.h>