Fix build for 9.x i386

Quarterly branch has a different version, but suffers from the same error, so
the fix should be backported.

videoframe.o: In function `VideoFrame::VideoFrame(unsigned long long, AVFrame*, QRect, int, bool)':
videoframe.cpp:(.text+0x19db): undefined reference to `atomic_fetch_add_8'
camerasource.o: In function `CameraSource::CameraSource()':
camerasource.cpp:(.text+0x327): undefined reference to `atomic_fetch_add_8'
corevideosource.o: In function `CoreVideoSource::CoreVideoSource()':
corevideosource.cpp:(.text+0x121): undefined reference to `__atomic_fetch_add_8'
collect2: error: ld returned 1 exit status
gmake: *** [Makefile:793: qtox] Error 1

PR: 213841
Submitted by: <matthew@reztek.cz>
Approved by: <yuri@rawbw.com> (maintainer)
MFH: 2016Q4