Add missing #include <cerrno> in string_to_number.cc

One of our toolchains does not expose |errno| in the global namespace.

BUG=none

Review-Url: https://codereview.webrtc.org/2926273002
Cr-Commit-Position: refs/heads/master@{#18506}
diff --git a/webrtc/base/string_to_number.cc b/webrtc/base/string_to_number.cc
index f880926..b8e969d 100644
--- a/webrtc/base/string_to_number.cc
+++ b/webrtc/base/string_to_number.cc
@@ -8,6 +8,7 @@
  *  be found in the AUTHORS file in the root of the source tree.
  */
 
+#include <cerrno>
 #include <cstdlib>
 
 #include "webrtc/base/string_to_number.h"