Split index xlog headers from other private index headers.

The xlog-specific headers need to be included in both frontend code -
specifically, pg_waldump - and the backend, but the remainder of the
private headers for each index are only needed by the backend.  By
splitting the xlog stuff out into separate headers, pg_waldump pulls
in fewer backend headers, which is a good thing.

Patch by me, reviewed by Michael Paquier and Andres Freund, per a
complaint from Dilip Kumar.

Discussion: http://postgr.es/m/CA+TgmoZ=F=GkxV0YEv-A8tb+AEGy_Qa7GSiJ8deBKFATnzfEug@mail.gmail.com