[compiler-rt] Include stdlib.h for exit() (#115025)

It was originally included transitively, but no longer is after recent
<vector> cleanups in libc++.

Similar to #113951.

Author: nico

compiler-rt/lib/fuzzer/FuzzerExtFunctionsWindows.cpp

@@ -14,6 +14,7 @@
 
 #include "FuzzerExtFunctions.h"
 #include "FuzzerIO.h"
+#include <stdlib.h>
 
 using namespace fuzzer;