GUI: Mark local flip() function as static

Change-Id: Ib92e294872c1408cc749fdb0db3bbf56579eb070
diff --git a/gui/gui.cpp b/gui/gui.cpp
index 3f1312f..369b6b8 100644
--- a/gui/gui.cpp
+++ b/gui/gui.cpp
@@ -83,7 +83,7 @@
 
 extern "C" void gr_write_frame_to_file(int fd);
 
-void flip(void)
+static void flip(void)
 {
 	if (gRecorder != -1)
 	{