Add missing dependency.

Teach Makefile that cmd.o depends on a generated header (specifically
config-host.h).
Signed-off-by: NPaul Brook <paul@codesourcery.com>

Makefile

@@ -150,7 +150,7 @@ version-obj-$(CONFIG_WIN32) += version.o
 ######################################################################
 
 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