v4l2object: Don't redefine mmap64

On Linux, there exist a case where mmap64 is already a define to mmap,
so avoid the redefine warning here.
diff --git a/sys/v4l2/gstv4l2object.c b/sys/v4l2/gstv4l2object.c
index 6c7a366..0ec774a 100644
--- a/sys/v4l2/gstv4l2object.c
+++ b/sys/v4l2/gstv4l2object.c
@@ -55,7 +55,7 @@
 
 #define ENCODED_BUFFER_SIZE             (2 * 1024 * 1024)
 
-#if SIZEOF_OFF_T == 8
+#if SIZEOF_OFF_T == 8 && !defined(mmap64)
 #define mmap64 mmap
 #endif