Add missing ThreadDefs.h header

Change 2016760 removed utils/threads.h from BpBinder.h
which indirectly included ThreadDefs.h. This change
adds ThreadDefs.h explicitly to vr_flinger.cpp which
needs it.

Bug: 223486215
Test: m
Merged-In: If4c6494d1bf3a32516ada3670ee449e2a047c7ed
Change-Id: I3be09bfd2820000129debb9f23f854df2a5703c2
1 file changed