[XRay][compiler-rt] Attempt to fix ARM buildbots after r287068

Include xray_defs.h in xray_arm.cc (seems to be the only one that doesn't
include it).

Buildbot errors:
[...]/compiler-rt/lib/xray/xray_arm.cc:31:58: error: expected initializer before 'XRAY_NEVER_INSTRUMENT'
 inline static uint32_t getMovwMask(const uint32_t Value) XRAY_NEVER_INSTRUMENT {

llvm-svn: 287089