dfu: fixup conditional CMake include

The dfu subsys is theoretically agnostic to the particular bootloader,
even if MCUboot is the only bootloader currently supported. Include the
dfu folder based on the parent symbol `IMG_MANAGER` instead of the
specific `MCUBOOT_IMG_MANAGER`.

The MCUboot specific files are already gated by the Kconfig in the
`boot` and `img_util` folders.

Signed-off-by: Jordan Yates <jordan.yates@data61.csiro.au>