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>
(cherry picked from commit 6e14404a)
Signed-off-by: NAurelien Jarno <aurelien@aurel32.net>

Makefile

@@ -107,7 +107,7 @@ bt-host.o: QEMU_CFLAGS += $(BLUEZ_CFLAGS)
 ######################################################################
 
 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 $(block-obj-y) $(qobject-obj-y)