meson: Specify -Wformat as a common warning flag for extensions

Hi,

I'm an extension developer. If I use PostgreSQL built with
Meson, I get the following warning:

    cc1: warning: '-Wformat-security' ignored without '-Wformat' [-Wformat-security]

Because "pg_config --cflags" includes -Wformat-security but
doesn't include -Wformat.

Can we specify -Wformat as a common warning flag too? If we
do it, "pg_config --cflags" includes both of
-Wformat-security and -Wformat. So I don't get the warning.


Thanks,
-- 
kou