package/snort3: link with -latomic if needed

package/snort3: link with -latomic if needed

Fix the following build failure raised since bump to version 3.1.6
in commit e66f2fd310374779d415fa683813cc5f5ccf6be9 and
https://github.com/snort3/snort3/commit/3e518d86040e74c328bcc0d5bbd49c1da6b7a3ec:

/home/buildroot/autobuild/instance-1/output-1/host/opt/ext-toolchain/bin/../lib/gcc/powerpc-buildroot-linux-uclibc/9.3.0/../../../../powerpc-buildroot-linux-uclibc/bin/ld: service_inspectors/dce_rpc/CMakeFiles/dce_rpc.dir/dce_smb2_file.cc.o: undefined reference to symbol '__atomic_load_8@@LIBATOMIC_1.0'

Fixes:
 - http://autobuild.buildroot.org/results/df34a69175e61bc7b180d89c738747e19aaf13bf

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