package/physfs: needs threads support

package/physfs: needs threads support

When physfs is built for a Linux system the PHYSFS_PLATFORM_POSIX (which
enable code that use pthread_*()) symbol must be defined, so threads support
is required.  The physfs build system used by the previous version didn't
correctly set PHYSFS_PLATFORM_POSIX for system without pthread support.

Add pthread dependency.

Fixes:
http://autobuild.buildroot.org/results/75d/75d68ff9da42d61f47d80b463445c12bc51ed1a4

Signed-off-by: Romain Naour <romain.naour@smile.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>