qt4: make pulseaudio support conditional upon DISTRO_FEATURES

qt4: make pulseaudio support conditional upon DISTRO_FEATURES

Add a pulseaudio DISTRO_FEATURES item and disable pulseaudio support in
phonon within Qt4 if it is not present; otherwise it is a race condition
as to whether it is enabled or disabled or breaks the build.

Note that this adds pulseaudio to DISTRO_FEATURES_BACKFILL thus leaving
existing distro configurations unaffected.

(From OE-Core rev: b82c216c1ee8e2a009e87856b7adad08f7f50482)

Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>