package/janet: link with -latomic if needed

Link with -latomic if needed to avoid the following build failure since
bump to version 1.18.1 in commit
1ffdab3e and
https://github.com/janet-lang/janet/commit/eb0b37f7296611a411b9a86d9d65fe620b0454d8:

/home/giuliobenetti/autobuild/run/instance-3/output-1/host/opt/ext-toolchain/bin/../lib/gcc/sparc-buildroot-linux-uclibc/9.3.0/../../../../sparc-buildroot-linux-uclibc/bin/ld: janet.p/meson-generated_.._janet.c.o: in function `janet_abstract_incref':
janet.c:(.text+0x2790): undefined reference to `__atomic_fetch_add_4'

Fixes:
 - http://autobuild.buildroot.org/results/8d204d8f065e6e4bc1376edd1f639864b3c69c0b



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