Unix/Signals.inc: Let findModulesAndOffsets() built conditionally regarding to (defined(HAVE_BACKTRACE) && defined(ENABLE_BACKTRACES)). [-Wunused-function]

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@219596 91177308-0d34-0410-b5e6-96231b3b80d8