Silence warning with MSVC.

Fixes:

[2587/4073] Building CXX object projects\compiler-rt\lib\sanitizer_common\CMakeFiles\RTSanitizerCommon.x86_64.dir\sanitizer_stoptheworld_win.cpp.obj
D:\git\llvm-project\compiler-rt\lib\sanitizer_common\sanitizer_stoptheworld_win.cpp(125,33): warning: comparison of integers of different signs: 'DWORD' (aka 'unsigned long') and 'int' [-Wsign-compare]
      if (SuspendThread(thread) == -1) {
          ~~~~~~~~~~~~~~~~~~~~~ ^  ~~
1 warning generated.