php: openssl support needs to link against zlib for static linking

Fixes:
http://autobuild.buildroot.net/results/490/490a9558635c90bf510b6f80cd0891fe322c3f46/
http://autobuild.buildroot.net/results/bf1/bf12eb189ab35ce00a2212695d2dbf8b8a126529/



Otherwise the configure script fails to detect openssl and bails out.

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>