Add missing dependency.

Teach Makefile that cmd.o depends on a generated header (specifically
config-host.h).

Signed-off-by: Paul Brook <paul@codesourcery.com>
diff --git a/Makefile b/Makefile
index 4e120a2..3389775 100644
--- a/Makefile
+++ b/Makefile
@@ -150,7 +150,7 @@
 ######################################################################
 
 qemu-img.o: qemu-img-cmds.h
-qemu-img.o qemu-tool.o qemu-nbd.o qemu-io.o: $(GENERATED_HEADERS)
+qemu-img.o qemu-tool.o qemu-nbd.o qemu-io.o cmd.o: $(GENERATED_HEADERS)
 
 qemu-img$(EXESUF): qemu-img.o qemu-tool.o qemu-error.o $(oslib-obj-y) $(trace-obj-y) $(block-obj-y) $(qobject-obj-y) $(version-obj-y) qemu-timer-common.o