pgstattuple: Add new error case for spgist indexes.

Extracted from a larger patch by Jaime Casanova, reviewed by Noah Misch.
I think this error message could use some more extensive revision, but
this at least makes the handling of spgist consistent with what we do for
other types of indexes that this code doesn't know how to handle.

contrib/pgstattuple/pgstattuple.c

@@ -231,6 +231,9 @@ pgstat_relation(Relation rel, FunctionCallInfo fcinfo)
 				case GIN_AM_OID:
 					err = "gin index";
 					break;
+				case SPGIST_AM_OID:
+					err = "spgist index";
+					break;
 				default:
 					err = "unknown index";
 					break;