meson: add external sensor to meson build

Add external to unit_files to make the service can be installed.

Tested: Add configuration for exteranlSensor and verify pass.

Signed-off-by: Jeff Lin <JeffLin2@quantatw.com>
Change-Id: I1ca650cc15d8191fdbecc25d5b5ba099f3ee1920
diff --git a/service_files/meson.build b/service_files/meson.build
index 64d0974..37bb261 100644
--- a/service_files/meson.build
+++ b/service_files/meson.build
@@ -9,6 +9,7 @@
     ['mcu', 'xyz.openbmc_project.mcutempsensor.service'],
     ['nvme', 'xyz.openbmc_project.nvmesensor.service'],
     ['psu', 'xyz.openbmc_project.psusensor.service'],
+    ['external', 'xyz.openbmc_project.externalsensor.service'],
 ]
 
 foreach tuple : unit_files