package/php: fix icu support

package/php: fix icu support

Propagate icu reverse dependencies.

Php lacks "-std=c++11", provided by icu-config, when compiling icu
support leading to a build failure:

/home/peko/autobuild/instance-1/output/host/usr/sh4-buildroot-linux-uclibc/sysroot/usr/include/unicode/umachine.h:347:13:
 error: 'char16_t' does not name a type
     typedef char16_t UChar;

Fixes
http://autobuild.buildroot.net/results/238/238d0e4bb205c0e7288903c6498e6c1787d86a1b/

Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>