package/php: ffi needs dynamic library

FFI support in php has been added with commit
e16f05c0, it depends on dynamic library
otherwise the build will fail on:

/home/buildroot/autobuild/instance-1/output-1/host/opt/ext-toolchain/bin/../lib/gcc/arm-buildroot-linux-uclibcgnueabi/5.5.0/../../../../arm-buildroot-linux-uclibcgnueabi/bin/ld: ext/ffi/ffi.o: in function `zim_FFI_cdef':
ffi.c:(.text+0xe78): undefined reference to `DL_LOAD'

Fixes:
 - http://autobuild.buildroot.org/results/3380b7b6777d1c5fcb53c855b003466fa3bf2079



Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>