gpu : add gpu sensor app

The commit adds a dbus sensor app that uses MCTP VDM protocol to read
temperature sensor value from the gpu.

The MCTP VDM protocol is an extension of the OCP specification -
'''
  https://www.opencompute.org/documents/ocp-gpu-accelerator-management-interfaces-v1-pdf
'''

Tested.

Copy the gpusensor app on gb200nvl-obmc machine and run it.
```
root@gb200nvl-obmc:~# ./gpusensor
```

The app runs without errors.
```
root@gb200nvl-obmc:~# busctl tree xyz.openbmc_project.GpuSensor
└─ /xyz
  └─ /xyz/openbmc_project
    └─ /xyz/openbmc_project/sensors
```

Change-Id: Iee7376a9116489052c690f2e3a1ca8d0f29564dd
Signed-off-by: Harshit Aghera <haghera@nvidia.com>
diff --git a/src/meson.build b/src/meson.build
index ca995e5..508bf85 100644
--- a/src/meson.build
+++ b/src/meson.build
@@ -111,6 +111,10 @@
     subdir('psu')
 endif
 
+if get_option('gpu').allowed()
+    subdir('gpu')
+endif
+
 if get_option('external').allowed()
     subdir('external')
 endif