php: fix iconv related build failure

Fix a rare edge build failure when iconv is enabled, easily reproduced
in a debian chroot with an aarch64 external toolchain with iconv +
xmlrpc support enabled with a php cgi + cli target.
Should solve bug #6500.

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>