video: Move video_get_info_str() prototype to a header file

This should be defined in a header file so that arguments are checked.
Move it to video.h.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Tom Rini <trini@konsulko.com>
diff --git a/board/liebherr/lwmon5/lwmon5.c b/board/liebherr/lwmon5/lwmon5.c
index 8ad6712..bb99a3b 100644
--- a/board/liebherr/lwmon5/lwmon5.c
+++ b/board/liebherr/lwmon5/lwmon5.c
@@ -13,6 +13,7 @@
 #include <asm/io.h>
 #include <post.h>
 #include <flash.h>
+#include <video.h>
 #include <mtd/cfi_flash.h>
 
 DECLARE_GLOBAL_DATA_PTR;