package/clamav: link with libatomic when needed

Configure check for OpenSSL fails:

/accts/mlweber1/rclinux/rc-buildroot-test/scripts/instance-3/output/host/sparc-buildroot-linux-uclibc/sysroot/usr/lib/libcrypto.a(threads_pthread.o): In function `CRYPTO_atomic_add':
threads_pthread.c:(.text+0x1dc): undefined reference to `__atomic_is_lock_free'
threads_pthread.c:(.text+0x1f4): undefined reference to `__atomic_fetch_add_4'

Fixes
http://autobuild.buildroot.net/results/cae8da81adff3ba493154e0ba8b21d90367f82eb/



Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>