Adds missing .php extension when including request-membership template in bp-default

This extension had been accidentally removed in r6517, with the result that
the request-membership template was not loaded in bp-default.

Fixes #5062

git-svn-id: https://buddypress.svn.wordpress.org/trunk@7230 cdf35c40-ae34-48e0-9cc9-0c9da1808c22
Pinking shears

git-svn-id: https://buddypress.svn.wordpress.org/trunk@7231 cdf35c40-ae34-48e0-9cc9-0c9da1808c22