message when eating a stalking

As suggested by <Someone> to the mailing list, display a message
when you eat a stalker and notice that you became invisible.

doc/fixes34.2

@@ -65,6 +65,7 @@ killer reason for named missile could end up with garbage instead of the name
 make killer reason for various poisioning deaths be more consistent
 poison missiles were unintentionally more likely to inflict "deadly poison"
 	than in pre-3.4.1 releases
+provide feedback when going invisible after eating a stalker
 
 
 Platform- and/or Interface-Specific Fixes

src/eat.c

@@ -820,6 +820,7 @@ register int pm;
 	    case PM_STALKER:
 		if(!Invis) {
 			set_itimeout(&HInvis, (long)rn1(100, 50));
+			if (!Blind && !BInvis) self_invis_message();
 		} else {
 			if (!(HInvis & INTRINSIC)) You_feel("hidden!");
 			HInvis |= FROMOUTSIDE;