configure: Add -lrt dependency to avutil's pc file.

This is needed for clock_gettime.

Signed-off-by: Luca Barbato <lu_zero@gentoo.org>
Merge commit '7d97511e13ef30cd99bae0e36400de055b72918d'

* commit '7d97511e13ef30cd99bae0e36400de055b72918d':
  configure: Add -lrt dependency to avutil's pc file.

Conflicts:
	configure

Merged-by: Michael Niedermayer <michaelni@gmx.at>
avformat/assdec: make sure pos is initialized

Fixes use of uninitialized memory
Fixes: signal_sigsegv_504fb0_10_signal_sigsegv_504fb0_343_mewmew_ssa.avi

Found-by: Mateusz j00ru Jurczyk and Gynvael Coldwind