Updated string.h

Summary:
@markisaa, @ldbrandy: stdarg.h is for ellipses arguments, ##...##, which this file does contain.
The pull request indicates that this header is required for windows. Given that adding it doesn't hurt, and should be there anyways, I'd say go on this pull request.

Fixed a fatal error when compiling folly for HHVM
Signed-off-by: Nicholas Ormrod <njormrod@fb.com>

Test Plan: contbuild

Reviewed By: markisaa@fb.com

Subscribers: folly-diffs@, yfeldblum, chalfant, ldbrandy, markisaa

FB internal diff: D1999557

Tasks: 6783581

Signature: t1:1999557:1429220813:0580882b8a55d86f51cdd602d2d152920d368b75

folly/String.h

@@ -18,6 +18,7 @@
 #define FOLLY_BASE_STRING_H_
 
 #include <exception>
+#include <stdarg.h>
 #include <string>
 #include <boost/type_traits.hpp>