libcutils: Add missing dependency on libprocessgroup_headers

cutils/sched_policy.h redirects to processgroup/sched_policy.h, but
libcutils does not export the libprocessgroup headers. So users have to
know about the hidden redirect to libprocessgroup and include a
libprocessgroup dependency in their Android.bp files in addition to
libcutils. Fix that.

Bug: 349105928
Test: m
Change-Id: I516bdf2cdeff980c1fcd18883ef8a2f0a9beb629