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>