meson: Enable warnings to flag missing declarations

A recently introduced typo resulted in a function definition not
matching its declaration. As the problem occurred in libipa, and IPA
modules are loaded dynamically and are linked with lazy symbol
resolution, the problem wasn't caught at build time.

To try and catch future similar issues, enable the
-Wmissing-declarations warning.

Suggested-by: Barnabás Pőcze <pobrn@protonmail.com>
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Kieran Bingham <kieran.bingham@ideasonboard.com>

meson.build

@@ -95,6 +95,7 @@ if cc.has_header_symbol('stdlib.h', 'secure_getenv', prefix : '#define _GNU_SOUR
 endif
 
 common_arguments = [
+    '-Wmissing-declarations',
     '-Wshadow',
     '-include', meson.current_build_dir() / 'config.h',
 ]