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>
diff --git a/subsys/CMakeLists.txt b/subsys/CMakeLists.txt
index 4318f31..a2f049f 100644
--- a/subsys/CMakeLists.txt
+++ b/subsys/CMakeLists.txt
@@ -12,7 +12,7 @@
 add_subdirectory(fs)
 add_subdirectory(ipc)
 add_subdirectory(mgmt)
-add_subdirectory_ifdef(CONFIG_MCUBOOT_IMG_MANAGER  dfu)
+add_subdirectory_ifdef(CONFIG_IMG_MANAGER          dfu)
 add_subdirectory_ifdef(CONFIG_NET_BUF              net)
 add_subdirectory(usb)
 add_subdirectory(random)