re2: #include <cstring> in prog.h

<cstring> provides memchr.

Change-Id: I7d6ef3d0f6c696d5a13353eb80b2a84f6e23fa7f
Reviewed-on: https://code-review.googlesource.com/c/re2/+/63772
Reviewed-by: Alan Donovan <adonovan@google.com>
diff --git a/re2/prog.h b/re2/prog.h
index d0889bd..801f8b9 100644
--- a/re2/prog.h
+++ b/re2/prog.h
@@ -11,6 +11,7 @@
 
 #include <stdint.h>
 
+#include <cstring>
 #include <functional>
 #include <string>
 #include <type_traits>