Makefile.user: Define CONFIG_USER_ONLY for libuser/

In *-*-user/ build dirs CONFIG_USER_ONLY is defined via config-target.h.
In libuser/ it is not defined.

Add it via QEMU_CFLAGS.

Signed-off-by: Andreas Färber <afaerber@suse.de>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>

Makefile.user

@@ -10,6 +10,7 @@ $(call set-vpath, $(SRC_PATH))
 
 QEMU_CFLAGS+=-I..
 QEMU_CFLAGS += -I$(SRC_PATH)/include
+QEMU_CFLAGS += -DCONFIG_USER_ONLY
 
 include $(SRC_PATH)/Makefile.objs