Add .gitignore entry for a derived file created by "make distprep".

I chanced to notice that "make distprep" leaves a state of the
tree that git complains about.  It's been like this for awhile,
but given the lack of complaints it probably doesn't need
back-patching.

src/interfaces/libpq/.gitignore

 /exports.list
 /libpq.rc
+/libpq-dist.rc
 # .c files that are symlinked in from elsewhere
 /encnames.c
 /wchar.c