Add -latomic to PKGCFG_LIBS_PRIVATE so applications linking statically
with libzmq (such czmq) will know that they have to link with -latomic
and the following build failure will be avoided:

  CCLD     src/czmq_selftest
/home/buildroot/autobuild/run/instance-3/output-1/host/opt/ext-toolchain/bin/../lib/gcc/sparc-buildroot-linux-uclibc/7.4.0/../../../../sparc-buildroot-linux-uclibc/bin/ld: /home/buildroot/autobuild/run/instance-3/output-1/host/sparc-buildroot-linux-uclibc/sysroot/usr/lib/libzmq.a(src_libzmq_la-ctx.o): in function `zmq::ctx_t::create_socket(int)':
ctx.cpp:(.text+0x1710): undefined reference to `__atomic_fetch_add_4'

Fixes:
 - http://autobuild.buildroot.org/results/4a12f1ede260cd956a0b5ccb4eec6ca8b44cb04fSigned-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>