Fix thinko: errmsg -> ereport.

Silly mistake in my commit 09cecdf285, reported by Erik Rijkers.

The fact that the buildfarm didn't find this implies that we are not
testing Perl builds that lack MULTIPLICITY, which is a bit disturbing
from a coverage standpoint.  Until today I'd have said nobody cared
about such configurations anymore; but maybe not.

src/pl/plperl/plperl.c

@@ -640,9 +640,9 @@ select_perl_context(bool trusted)
 		else
 			plperl_untrusted_init();
 #else
-		errmsg(ERROR,
-			   (errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
-				errmsg("cannot allocate multiple Perl interpreters on this platform")));
+		ereport(ERROR,
+				(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
+				 errmsg("cannot allocate multiple Perl interpreters on this platform")));
 #endif
 	}