Makefile: make qemu-img dependant on config-host.h

Since config-host.h is generated by the Makefile (1215c6e7),
building (only) qemu-img fails:

[user@f12-uri qemu]$ make distclean (or git clone qemu)
[user@f12-uri qemu]$ ./configure ...
[user@f12-uri qemu]$ make qemu-img
GEN  config-all-devices.mak
GEN   qemu-img-cmds.h
CC    qemu-img.o
In file included from qemu-img.c:24:
qemu-common.h:32:25: error: config-host.h: No such file or directory
Signed-off-by: NUri Lublin <uril@redhat.com>
Signed-off-by: NAnthony Liguori <aliguori@us.ibm.com>

Makefile

@@ -209,7 +209,7 @@ libqemu_common.a: $(obj-y)
 
 ######################################################################
 
-qemu-img.o: qemu-img-cmds.h
+qemu-img.o: config-host.h qemu-img-cmds.h
 
 qemu-img$(EXESUF): qemu-img.o qemu-tool.o $(block-obj-y)