[clang] FatalErrorHandler.cpp - add explicit <stdio.h> include

Required for fprintf/stderr usage in the error handler, noticed while trying to remove the <string> dependency described in D111049

clang/tools/libclang/FatalErrorHandler.cpp

@@ -9,6 +9,7 @@
 
 #include "clang-c/FatalErrorHandler.h"
 #include "llvm/Support/ErrorHandling.h"
+#include <stdio.h>
 #include <stdlib.h>
 
 static void aborting_fatal_error_handler(void *, const char *reason,