tools: iio: Correctly add make dependency for iio_utils

iio tools fail to build correctly with make parallelization:

$ make -s -j24
fixdep: error opening depfile: ./.iio_utils.o.d: No such file or directory
make[1]: *** [/home/labbott/linux_upstream/tools/build/Makefile.build:96: iio_utils.o] Error 2
make: *** [Makefile:43: iio_event_monitor-in.o] Error 2
make: *** Waiting for unfinished jobs....

This is because iio_utils.o is used across multiple targets.
Fix this by making iio_utils.o a proper dependency.

Signed-off-by: Laura Abbott <labbott@redhat.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>