code-style: add fallthrough comment

Patch by Steve Ward, thanks.

st.c

@@ -2153,6 +2153,7 @@ tcontrolcode(uchar ascii)
 		return;
 	case '\032': /* SUB */
 		tsetchar('?', &term.c.attr, term.c.x, term.c.y);
+		/* FALLTHROUGH */
 	case '\030': /* CAN */
 		csireset();
 		break;

x.c

@@ -1528,6 +1528,7 @@ xdrawcursor(int cx, int cy, Glyph g, int ox, int oy, Glyph og)
 		switch (win.cursor) {
 		case 7: /* st extension: snowman (U+2603) */
 			g.u = 0x2603;
+			/* FALLTHROUGH */
 		case 0: /* Blinking Block */
 		case 1: /* Blinking Block (Default) */
 		case 2: /* Steady Block */