Makefile.am: distribute systemd/mpd.socket

The file systemd/mpd.service.in is being distributed implicitly, but
systemd/mpd.socket is not and needs to be added to EXTRA_DIST.

Makefile.am

@@ -2194,4 +2194,5 @@ EXTRA_DIST = $(doc_DATA) autogen.sh \
 	test/test_archive_zzip.sh \
 	$(wildcard scripts/*.sh) \
 	$(man_MANS) $(DOCBOOK_FILES) doc/mpdconf.example doc/doxygen.conf \
+	systemd/mpd.socket \
 	src/win32/mpd_win32_rc.rc.in src/win32/mpd.ico

NEWS

@@ -2,6 +2,7 @@ ver 0.19.1 (not yet released)
 * input
   - mms: fix deadlock bug
 * fix build failure on Mac OS X
+* add missing file systemd/mpd.socket to tarball
 
 ver 0.19 (2014/10/10)
 * protocol