tools: mkimage: Mark copy_datafile() as static

The function is only used locally. Enabling -Wmissing-prototypes
triggers a warning. Mark it as static.

Signed-off-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>

tools/mkimage.c

@@ -445,7 +445,7 @@ static void verify_image(const struct image_type_params *tparams)
 	(void)close(ifd);
 }
 
-void copy_datafile(int ifd, char *file)
+static void copy_datafile(int ifd, char *file)
 {
 	if (!file)
 		return;