add missing case for D_DCR to allow MOVW to and from DCR(n) to work

Reference: /n/sources/patch/applied/qa-dcr
Date: Mon Sep 11 08:55:42 CES 2006
Signed-off-by: forsyth@terzarima.net

--- /sys/src/cmd/qa/lex.c	Mon Sep 11 08:54:55 2006
+++ /sys/src/cmd/qa/lex.c	Mon Sep 11 08:54:52 2006
@@ -662,6 +662,7 @@
 	case D_OPT:
 		break;
 
+	case D_DCR:
 	case D_SPR:
 	case D_OREG:
 	case D_CONST: