tools: mkenvimage: Make xstrtol() static

This 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/mkenvimage.c

@@ -47,7 +47,7 @@ static void usage(const char *exec_name)
 	       exec_name);
 }
 
-long int xstrtol(const char *s)
+static long int xstrtol(const char *s)
 {
 	long int tmp;