package/php: fix build without zlib

Build will fail if zlib is not found and mysqlnd compression support
is not disabled since version 7.4.1 and
https://github.com/php/php-src/commit/ee4295b4ce421003c2e1d2af98066826deb23319

Fixes:
 - http://autobuild.buildroot.org/results/9496d81437dba55d22a03762dcfe60d632115ab5



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