presence: Add fan utility

Add utility wrappers for reading/writing
xyz.openbmc_project.Inventory.Item.Present.

Change-Id: I9d6bb776e56b67b9cac1f991790886ec8105d4b1
Signed-off-by: Brad Bishop <bradleyb@fuzziesquirrel.com>
diff --git a/presence/Makefile.am b/presence/Makefile.am
index c3c32a5..e06a135 100644
--- a/presence/Makefile.am
+++ b/presence/Makefile.am
@@ -5,6 +5,7 @@
 	phosphor-fan-presence-tach
 
 phosphor_fan_presence_tach_SOURCES = \
+	fan.cpp \
 	fan_enclosure.cpp \
 	tach_sensor.cpp \
 	tach_detect.cpp