package/gqrx: link with libatomic when needed

On some architectures, atomic builtins are provided by the libatomic
library from gcc. Linking with libatomic is therefore necessary,
otherwise the build fails with:

sparc-buildroot-linux-uclibc/sysroot/lib/libatomic.so.1: error adding symbols: DSO missing from command line

Fixes:
 - http://autobuild.buildroot.net/results/a003ad5324a0c0f55cb8db5d3e5d69bd21999e16/
 - http://autobuild.buildroot.net/results/464602175d026d135125e5baa00e0729aec7a931/



Signed-off-by: Gwenhael Goavec-Merou <gwenhael.goavec-merou@trabucayre.com>
[Peter: add dependency on sync or atomic builtins in Config, addd comment]
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>