x11/xdm: Fix generation of etc/X11/xdm/Xresources

By some stupidity of autotools, spacing for a sed regexp used to generate
Xresources gets broken, and the sed command doen't match as it should. This
results in an Xresources file with an extra '#endif /* XPM */', which breaks
the file.
Patch Makefile.in to fix the regexp and have Xresources generated properly.

PR: 244404
Reported by: olgeni
MFH: 2020Q1