etc/: remove systemd unit files on clean

Fix 'make distcheck'
ERROR: files left in build directory after distclean:
./etc/pcscd.socket
./etc/pcscd.service
make[1]: *** [Makefile:732 : distcleancheck] Erreur 1

Author: LudovicRousseau

etc/Makefile.am

@@ -7,6 +7,8 @@ systemdsystemunit_DATA = \
 	pcscd.service \
 	pcscd.socket
 
+CLEANFILES = $(systemdsystemunit_DATA)
+
 edit = sed \
 	-e 's|@sysconfdir[@]|$(sysconfdir)|g' \
 	-e 's|@sbindir_exp[@]|$(sbindir_exp)|g' \