libavb: Fix a warning with clang-15

With clang-15 we now will get warnings such as:

warning: a function declaration without a prototype is deprecated in all
versions of C [-Wstrict-prototypes]

And it is easy enough to address this warning here, as we aren't
concerned with re-syncing with an upstream.

Signed-off-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Simon Glass <sjg@chromium.org>

lib/libavb/avb_cmdline.c

@@ -394,7 +394,7 @@ out:
   return ret;
 }
 
-AvbCmdlineSubstList* avb_new_cmdline_subst_list() {
+AvbCmdlineSubstList* avb_new_cmdline_subst_list(void) {
   return (AvbCmdlineSubstList*)avb_calloc(sizeof(AvbCmdlineSubstList));
 }