Build usb-ohci for PCs

The OHCI emulation isn't obviously broken and there are people who want to use
it. Let's build it by default so that it can be enabled via -device.
Signed-off-by: NKevin Wolf <kwolf@redhat.com>
Signed-off-by: NAurelien Jarno <aurelien@aurel32.net>

default-configs/i386-softmmu.mak

 # Default configuration for i386-softmmu
+
+CONFIG_USB_OHCI=y

default-configs/x86_64-softmmu.mak

 # Default configuration for x86_64-softmmu
+
+CONFIG_USB_OHCI=y