Add mapper dependency to service file

When the application starts it makes a GetObject method call on the
mapper, so add the dependency to the service file to ensure the mapper
starts first.

Change-Id: I9364241606c350ac4ca859e00c2587092db417d1
Signed-off-by: Matt Spinler <spinler@us.ibm.com>
diff --git a/phosphor-virtual-sensor.service.in b/phosphor-virtual-sensor.service.in
index 4bba079..c687e09 100644
--- a/phosphor-virtual-sensor.service.in
+++ b/phosphor-virtual-sensor.service.in
@@ -1,5 +1,7 @@
 [Unit]
 Description=Virtual sensors
+Wants=obmc-mapper.target
+After=obmc-mapper.target
 
 [Service]
 ExecStart=@bindir@/virtual-sensor