package/squid: needs atomic

package/squid: needs atomic

Since https://github.com/squid-cache/squid/commit/4b0f89121135aae68fbaf2aa33b5fb2e0da66d3e
squid tries to find if latomic is needed through:
AC_SEARCH_LIBS([__atomic_load_8],[atomic],[ATOMICLIB="-latomic"],[])

However, this can fails on:
configure:21147: /home/fabrice/buildroot/output/host/bin/arc-buildroot-linux-gnu-g++ -o conftest -D_LARGEFILE_SOURCE -D_LARGEFILE64_SOURCE -D_FILE_OFFSET_BITS=64 -matomic -Os   -D_LARGEFILE_SOURCE -D_LARGEFILE64_SOURCE -D_FILE_OFFSET_BITS=64 -g conftest.cpp -latomic   >&5
conftest.cpp:55:6: error: new declaration 'char __atomic_load_8()' ambiguates built-in declaration 'long long unsigned int __atomic_load_8(const volatile void*, int)' [-fpermissive]
 char __atomic_load_8 ();
      ^~~~~~~~~~~~~~~
conftest.cpp: In function 'int main()':
conftest.cpp:59:25: error: too few arguments to function 'long long unsigned int __atomic_load_8(const volatile void*, int)'
 return __atomic_load_8 ();

So add -latomic to LIBS if BR2_TOOLCHAIN_HAS_LIBATOMIC is set

Fixes:
 - http://autobuild.buildroot.org/results/13082cea836a12ac8bf85cbdb53a56a5d30c70b1

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>