Add more files to gitignore

When cloning a fresh copy of GPDB, running through the documented make
process, and then running the make target for the demo cluster, there
are three files that get generated. This commit adds those files to the
.gitignore files in their respective directories.
Authored-by: NJim Doty <jdoty@pivotal.io>

gpAux/gpdemo/.gitignore

+clusterConfigPostgresAddonsFile
 clusterConfigFile
 hostfile
 datadirs

src/backend/catalog/.gitignore

@@ -3,3 +3,4 @@
 /postgres.shdescription
 /cdb_init.sql
 /schemapg.h
+/pg_proc_combined.h

src/backend/utils/.gitignore

@@ -2,4 +2,5 @@
 /fmgroids.h
 /probes.h
 /errcodes.h
+/pg_proc_combined.h
 *.tmp