include: Don't redefine _GNU_SOURCE

[André Hentschel]

Otherwise a warning is triggered on Android:
../../../include/wine/port.h:32:0: warning: "_GNU_SOURCE" redefined [enabled by default]

---
 include/wine/port.h | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/include/wine/port.h b/include/wine/port.h
index 09dbf61..c5359dd 100644
--- a/include/wine/port.h
+++ b/include/wine/port.h
@@ -29,7 +29,9 @@
 # error You must include port.h before all other headers
 #endif
 
-#define _GNU_SOURCE  /* for pread/pwrite */
+#ifndef _GNU_SOURCE
+# define _GNU_SOURCE  /* for pread/pwrite */
+#endif
 #include <fcntl.h>
 #include <math.h>
 #include <sys/types.h>
-- 
1.8.1.2