pulse: Add a wallclock option to be compatible with other other captures

alsa and x11grab use av_gettime() to report timestamps.

Have it on by default.

Bug-Id: 647
(cherry picked from commit 424b929b5cb9ca4094099f25179829260d4b0fa3)
(cherry picked from commit 404731bd20e1df5880e6fe381e975ba48afc75b2)
Signed-off-by: Luca Barbato <lu_zero@gentoo.org>