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 <[e-mail]>
Signed-off-by: Jonathan Cameron <[e-mail]>

tools/iio/Build

···
+iio_utils-y += iio_utils.o
 lsiio-y += lsiio.o iio_utils.o
 iio_event_monitor-y += iio_event_monitor.o iio_utils.o
 iio_generic_buffer-y += iio_generic_buffer.o iio_utils.o

tools/iio/Makefile

···
 
 prepare: $(OUTPUT)include/linux/iio
 
+IIO_UTILS_IN := $(OUTPUT)iio_utils-in.o
+$(IIO_UTILS_IN): prepare FORCE
+	$(Q)$(MAKE) $(build)=iio_utils
+
 LSIIO_IN := $(OUTPUT)lsiio-in.o
-$(LSIIO_IN): prepare FORCE
+$(LSIIO_IN): prepare FORCE $(OUTPUT)iio_utils-in.o
 	$(Q)$(MAKE) $(build)=lsiio
 $(OUTPUT)lsiio: $(LSIIO_IN)
 	$(QUIET_LINK)$(CC) $(CFLAGS) $(LDFLAGS) $< -o $@
 
 IIO_EVENT_MONITOR_IN := $(OUTPUT)iio_event_monitor-in.o
-$(IIO_EVENT_MONITOR_IN): prepare FORCE
+$(IIO_EVENT_MONITOR_IN): prepare FORCE $(OUTPUT)iio_utils-in.o
 	$(Q)$(MAKE) $(build)=iio_event_monitor
 $(OUTPUT)iio_event_monitor: $(IIO_EVENT_MONITOR_IN)
 	$(QUIET_LINK)$(CC) $(CFLAGS) $(LDFLAGS) $< -o $@
 
 IIO_GENERIC_BUFFER_IN := $(OUTPUT)iio_generic_buffer-in.o
-$(IIO_GENERIC_BUFFER_IN): prepare FORCE
+$(IIO_GENERIC_BUFFER_IN): prepare FORCE $(OUTPUT)iio_utils-in.o
 	$(Q)$(MAKE) $(build)=iio_generic_buffer
 $(OUTPUT)iio_generic_buffer: $(IIO_GENERIC_BUFFER_IN)
 	$(QUIET_LINK)$(CC) $(CFLAGS) $(LDFLAGS) $< -o $@