Makefile.incl1: .WAIT before distribute in etc

In order to make sure that man pages are all installed before we run
makewhatis to generate mandoc.db files, we have long placed etc at the
end of the list of subdirectories being recursed into by the build.
In order to support installworld -jN, a .WAIT was more recently added
here.

With the recent adoption by the release engineering team of parallel
*release* builds (aka 'make release -jN') it is now also necessary to
add the same .WAIT before recursing for the 'distribute' target, as we
otherwise end up with distribution sets containing incomplete mandoc.db
files.

Reviewed by: bdrewery
PR: 289683
MFC after: 3 days
Sponsored by: https://www.patreon.com/cperciva
Differential Revision: https://reviews.freebsd.org/D53533