php: fix C++ link issue

The intl module is implemented in C++, but PHP fails to use g++ as the
compiler for the final link. As a workaround, tell it to link
with libstdc++.

Fixes:

  http://autobuild.buildroot.org/results/13eefdf2121fa1aea7a844bbed8b6cbce3b45996/build-end.log



Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>