php: switch to pcre2

php: switch to pcre2

php moved from pcre to pcre2 since bump to version 7.3 and
http://github.com/php/php-src/commit/a5bc5aed71f7a15f14f33bb31b8e17bf5f327e2d

This fixes a build failure: without this change, if BR2_PACKAGE_PCRE is
set, external pcre support in php is (wrongly) enabled with
--with-pcre-regex but because pcre2 was not found, php fallbacks on
built-in pcre2 without the "SLJIT_SINGLE_THREADED hack"

Fixes:
 - http://autobuild.buildroot.org/results/40ef339019203d2cc49d388e222cf17c3ca37944

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>