cmake: ensure WITH_AIO=0 and WITH_IIOD_USBD=1 fails

at cmake time, not at link time.

Turns this:
/usr/bin/ld: CMakeFiles/iiod.dir/iiod.c.o: in function `main':
/home/benja/git/libiio/iiod/iiod.c:531: undefined reference to `start_usb_daemon'
[100%] Built target iio_stresstest
collect2: error: ld returned 1 exit status
make[2]: *** [iiod/CMakeFiles/iiod.dir/build.make:227: iiod/iiod] Error 1
make[1]: *** [CMakeFiles/Makefile2:466: iiod/CMakeFiles/iiod.dir/all] Error 2
make[1]: *** Waiting for unfinished jobs....
[100%] Built target iio_attr
make: *** [Makefile:136: all] Error 2

into this:
CMake Error at iiod/CMakeLists.txt:72 (message):
  Sorry, USB support in iiod (WITH_IIOD_USBD) requires async I/O support
  (WITH_AIO)

should resolve analogdevicesinc#984

Signed-off-by: Robin Getz <rgetz@mathworks.com>

iiod/CMakeLists.txt

@@ -68,6 +68,9 @@ if (WITH_AIO)
 
 	target_include_directories(iiod PRIVATE ${LIBAIO_INCLUDE_DIR})
 	target_link_libraries(iiod LINK_PRIVATE ${LIBAIO_LIBRARIES})
+elseif (WITH_IIOD_USBD)
+	message(SEND_ERROR "Sorry, USB support in iiod (WITH_IIOD_USBD)"
+		" requires async I/O support (WITH_AIO)")
 endif()
 
 option(WITH_IIOD_SERIAL "Add serial (UART) support" ON)