net/cpp-pcp-client: fix build on GCC-based systems
by adding compiler:c++11-lang to USES:
cc1plus: error: unrecognized command line option "-Wno-unused-local-typedefs" cc1plus: error: unrecognized command line option "-Wno-maybe-uninitialized" cc1plus: error: unrecognized command line option "-std=c++11"