package/php: Always link with libpthread for static builds

Fixes
http://autobuild.buildroot.net/results/c9f/c9fb2b4a25817d6455472767819018a62281d5f9/
http://autobuild.buildroot.net/results/a75/a757edcfce00d3e52c6787f28dc31cbaf8d2d2ab/
http://autobuild.buildroot.net/results/8df/8df836b6c241954449544c7b4c74a1cb19e7ff7b/
http://autobuild.buildroot.net/results/1b6/1b6e571e307c2b190116601bade382c43e8d3858/
http://autobuild.buildroot.net/results/aa3/aa34b2326a0702093162eb1f9d7bdf9c7cf45311/
http://autobuild.buildroot.net/results/74a/74ae21d78f7ea9d71407accd3e43900af6766e68/
http://autobuild.buildroot.net/results/7ec/7ec28c23b3f12e5b7e2376c7913329d2a38dd232/



Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Acked-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>