Build: fix "make dist" in one go deficiency (and if, it's brokenness)

Problem was with EXTRA_DIST variable that got rightfully assigned
"$(docbook:%=%.xml)" in e99ccd1b0, but frightfully stayed unadjusted
in 7382e6241 -- actually it sort of worked, but only as a byproduct
of generating spurious empty <publication dir>.xml files, which
moreover required multiple iteration over "make dist" for it to
finish successfully -- in a limited sense of "successfully" since
it didn't include the publications' sources at all.

All this is now rectified. Also note that preceding "Build: configure:
let "make dist" tarballs be posix/pax, not v7 format" commit prepared
the path for us, otherwise we'd be getting something like:

pacemaker-<GITHASH>/doc/Pacemaker_Administration/en-US/Ch-Upgrading.txt:

file name is too long (max 99); not dumped