meson: add -Wall and -Wextra explicitly

meson: add -Wall and -Wextra explicitly

If someone runs `meson setup --buildtype plain` meson ignores
warning_level=2 that is in our meson.build file. The implication is
that Meson will not automatically add -Wall which enables -Wformat.

This breaks building libvirt from git with the buildtype set to plain.

There is an issue reported [1] to not ignore warning_level silently
and the change to ignore it was done by upstream commit [2].

This change makes Meson print warnings about using warning_level which
as described above doesn't work for all build types.

[1] <https://github.com/mesonbuild/meson/issues/7399>
[2] <https://github.com/mesonbuild/meson/commit/8ee1c9a07a3a35e3ed262fbc358fd86c257a966e>

Signed-off-by: Pavel Hrdina <phrdina@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>