pulseaudio: add m4-native to DEPENDS

Normally m4-native ends up in the sysroot via the toolchain, but if a
non-standard toolchain is used them m4-native may not be installed.

However Pulseaudio explicitly checks for m4 in the meson.build, so add
it to DEPENDS.

Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>