package/ffmpeg: add -latomic to pkg-config files

package/ffmpeg: add -latomic to pkg-config files

-latomic was added to extralibs to fix static build of ffmpeg in commit
fc8798197b57488a852e2c14b4677b5eb0114a0b. However, extralibs is not
added to libavformat.pc resulting in the following static build failure
of motion:

/home/buildroot/autobuild/instance-1/output-1/host/opt/ext-toolchain/bin/../lib/gcc/arm-buildroot-linux-uclibcgnueabi/9.3.0/../../../../arm-buildroot-linux-uclibcgnueabi/bin/ld: /home/buildroot/autobuild/instance-1/output-1/host/bin/../arm-buildroot-linux-uclibcgnueabi/sysroot/usr/lib/libavformat.a(fifo.o): in function `fifo_init':
/home/buildroot/autobuild/instance-1/output-1/build/ffmpeg-4.4/libavformat/fifo.c:519: undefined reference to `__atomic_store_8'

So add a patch to add extralibs (and so -latomic) to all pkg-config
files

Fixes:
 - http://autobuild.buildroot.org/results/62ec618e40081a250b8129ec6f5a178eb06fba1d

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>