Start removing #include directives from util/util.h.

Of course, this entails adding them to individual source files.

Change-Id: Iafa89975cfa736ac4c63559ec5e8ec88d4d97eb4
Reviewed-on: https://code-review.googlesource.com/5441
Reviewed-by: Paul Wankadia <[email protected]>
diff --git a/re2/set.h b/re2/set.h
index 6efc972..66366b2 100644
--- a/re2/set.h
+++ b/re2/set.h
@@ -5,7 +5,7 @@
 #ifndef RE2_SET_H_
 #define RE2_SET_H_
 
-#include <utility>
+#include <string>
 #include <vector>
 
 #include "re2/re2.h"