[Support] Add missing <cstdint> header to Signals.h

Without the change llvm build fails on this week's gcc-13 snapshot as:

    [  0%] Building CXX object lib/Support/CMakeFiles/LLVMSupport.dir/Signals.cpp.o
    In file included from llvm/lib/Support/Signals.cpp:14:
    llvm/include/llvm/Support/Signals.h:119:8: error: variable or field 'CleanupOnSignal' declared void
      119 |   void CleanupOnSignal(uintptr_t Context);
          |        ^~~~~~~~~~~~~~~

(cherry picked from commit ff1681dd)