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/tostring.cc b/re2/tostring.cc
index 0230c8c..b967198 100644
--- a/re2/tostring.cc
+++ b/re2/tostring.cc
@@ -5,6 +5,9 @@
 // Format a regular expression structure as a string.
 // Tested by parse_test.cc
 
+#include <string.h>
+#include <string>
+
 #include "util/util.h"
 #include "re2/regexp.h"
 #include "re2/walker-inl.h"