package/libmediaart: link with -latomic if needed

Fix the following build failure with BR2_PACKAGE_LIBMEDIAART_BACKEND_QT
raised since bump to version 1.9.5 in commit
3db4d486:

/home/buildroot/autobuild/instance-0/output-1/host/opt/ext-toolchain/bin/../lib/gcc/sparc-buildroot-linux-uclibc/10.3.0/../../../../sparc-buildroot-linux-uclibc/bin/ld: libmediaart/libmediaart-2.0.so.0.905.0.p/extractqt.cpp.o: in function `std::__atomic_base<int>::operator--()':
/home/buildroot/autobuild/instance-0/output-1/host/opt/ext-toolchain/sparc-buildroot-linux-uclibc/include/c++/10.3.0/bits/atomic_base.h:334: undefined reference to `__atomic_fetch_sub_4'

Fixes:
 - http://autobuild.buildroot.org/results/39aa4ecd37de774e06f67a6e1c4495dc1f0917b4



Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>