jsonescape.cc: Add missing header includes.

git-svn-id: file:///home/xapian-svn/xapian/trunk@17044 6b2f1b92-a4f3-0310-943f-de3cbe41748b
Makefile.am: Ship jsonescape.h.

git-svn-id: file:///home/xapian-svn/xapian/trunk@17045 6b2f1b92-a4f3-0310-943f-de3cbe41748b