nvramcui: Make local render_form() function static

Allows us to build with `-Wmissing-prototypes`.

Change-Id: I722b41e515ee472697028a912b9136ce59611051
Signed-off-by: Nico Huber <nico.h@gmx.de>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/47635
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>
Reviewed-by: Angel Pons <th3fanbus@gmail.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

payloads/nvramcui/nvramcui.c

@@ -37,7 +37,7 @@ static int max(int x, int y)
 	return y;
 }
 
-void render_form(FORM *form)
+static void render_form(FORM *form)
 {
 	int y, x, line;
 	WINDOW *w = form_win(form);