Fix dumb typo in SECURITY LABEL error message.

Report by Peter Eisentraut.

src/backend/commands/seclabel.c

@@ -62,7 +62,7 @@ ExecSecLabelStmt(SecLabelStmt *stmt)
 		if (label_provider_list == NIL)
 			ereport(ERROR,
 					(errcode(ERRCODE_INVALID_PARAMETER_VALUE),
-				 errmsg("security label providers have been loaded")));
+				 errmsg("no security label providers have been loaded")));
 		if (lnext(list_head(label_provider_list)) != NULL)
 			ereport(ERROR,
 					(errcode(ERRCODE_INVALID_PARAMETER_VALUE),

src/test/regress/output/security_label.source

@@ -22,13 +22,13 @@ RESET client_min_messages;
 -- Test of SECURITY LABEL statement without a plugin
 --
 SECURITY LABEL ON TABLE seclabel_tbl1 IS 'classified';			-- fail
-ERROR:  security label providers have been loaded
+ERROR:  no security label providers have been loaded
 SECURITY LABEL FOR 'dummy' ON TABLE seclabel_tbl1 IS 'classified';		-- fail
 ERROR:  security label provider "dummy" is not loaded
 SECURITY LABEL ON TABLE seclabel_tbl1 IS '...invalid label...';		-- fail
-ERROR:  security label providers have been loaded
+ERROR:  no security label providers have been loaded
 SECURITY LABEL ON TABLE seclabel_tbl3 IS 'unclassified';			-- fail
-ERROR:  security label providers have been loaded
+ERROR:  no security label providers have been loaded
 -- Load dummy external security provider
 LOAD '@abs_builddir@/dummy_seclabel@DLSUFFIX@';
 --