build: Fix 'make dist' regression

Commit f084b603

 incorrectly set
DIST_SUBDIRS for the toplevel Makefile.am.  In general actually we
don't need to set it, because modern automake automatically sets
it by looking at conditionals for SUBDIRS.

Tested-by: Rico Tzschichholz <ricotz@t-online.de>

https://bugzilla.gnome.org/show_bug.cgi?id=667806