win: include stdlib.h for mbstowcs_s, wcstombs_s

This used to come in transitively via <vector>, but upstream libc++
cleaned up its <vector> a bit and this is no longer true with new
libc++ versions.

No behavior change.

Bug: chromium:376278210
Change-Id: I658f500cfd0e3a33c260d34302557b3ceb2799ca
Reviewed-on: https://chromium-review.googlesource.com/c/breakpad/breakpad/+/5994352
Reviewed-by: Mark Mentovai <mark@chromium.org>

src/common/windows/string_utils.cc

@@ -30,6 +30,8 @@
 #include <config.h>  // Must come first
 #endif
 
+#include <stdlib.h>
+
 #include <cassert>
 #include <vector>