package/suricata: link with libatomic if needed

Fix build of suricata 6.0.0 with mips32r6

app-layer-ftp.o: In function `FTPCheckMemcap':
app-layer-ftp.c:(.text+0x284): undefined reference to `__atomic_load_8'
app-layer-ftp.c:(.text+0x2d8): undefined reference to `__atomic_fetch_add_8'

Fixes:
 - http://autobuild.buildroot.org/results/f574005204905250702df32b61c85d427ab4feda



Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>