PL/Tcl: Fix compiler warnings with Tcl 8.6

Some constification was added in the Tcl APIs, so add the modifiers in
PL/Tcl as well.

src/pl/tcl/pltcl.c

@@ -18,6 +18,11 @@
 #define CONST84
 #endif
 
+/* ... and for Tcl 8.6. */
+#ifndef CONST86
+#define CONST86
+#endif
+
 #include "access/htup_details.h"
 #include "access/xact.h"
 #include "catalog/pg_proc.h"
@@ -259,7 +264,7 @@ pltcl_FinalizeNotifier(ClientData clientData)
 }
 
 static void
-pltcl_SetTimer(Tcl_Time *timePtr)
+pltcl_SetTimer(CONST86 Tcl_Time *timePtr)
 {
 }
 
@@ -285,7 +290,7 @@ pltcl_ServiceModeHook(int mode)
 }
 
 static int
-pltcl_WaitForEvent(Tcl_Time *timePtr)
+pltcl_WaitForEvent(CONST86 Tcl_Time *timePtr)
 {
 	return 0;
 }