native_posix: kconfig: Fix misplaced USB_NATIVE_POSIX default

Should be within the 'if BOARD_NATIVE_POSIX`, or USB_NATIVE_POSIX will
get enabled whenever USB is (unless a user value overrides it).

Probably didn't cause any problems, since
boards/posix/native_posix/Kconfig.defconfig is only included for this
board.

Signed-off-by: Ulf Magnusson <Ulf.Magnusson@nordicsemi.no>