fix dependencies for snap-stage[23]

Author: nikomatsakis

mk/snap.mk

@@ -2,8 +2,8 @@
 snap-stage1: $(HSREQ1_H_$(CFG_HOST_TRIPLE))
 	$(S)src/etc/make-snapshot.py stage1 $(CFG_HOST_TRIPLE)
 
-snap-stage2: $(HSREQ2_H_$(CFG_HOST_TRIPLE)
+snap-stage2: $(HSREQ2_H_$(CFG_HOST_TRIPLE))
 	$(S)src/etc/make-snapshot.py stage2 $(CFG_HOST_TRIPLE)
 
-snap-stage3: $(HSREQ3_H_$(CFG_HOST_TRIPLE)
+snap-stage3: $(HSREQ3_H_$(CFG_HOST_TRIPLE))
 	$(S)src/etc/make-snapshot.py stage3 $(CFG_HOST_TRIPLE)