package/sysrepo: link with libatomic when needed

package/sysrepo: link with libatomic when needed

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

/home/buildroot/autobuild/instance-2/output-1/host/opt/ext-toolchain/bin/../lib/gcc/sparc-buildroot-linux-uclibc/7.4.0/../../../../sparc-buildroot-linux-uclibc/bin/ld: libsysrepo.so.0.7.8: undefined reference to `__atomic_fetch_add_4'

This is often for example the case on sparcv8 32 bit.

Fixes:
 - http://autobuild.buildroot.org/results/d807f3dc922a61e099a66de3bc340c018a86e150

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