Add mts tag to the build file for MTS testing

Bug: 145490751
Test: build, atest CtsTetheringTest
Change-Id: I993ce5ec89ab98de7eadaf6624a30a37851ca716
diff --git a/tests/cts/tethering/Android.bp b/tests/cts/tethering/Android.bp
index 7b16879..0f98125 100644
--- a/tests/cts/tethering/Android.bp
+++ b/tests/cts/tethering/Android.bp
@@ -38,6 +38,7 @@
     test_suites: [
         "cts",
         "general-tests",
+        "mts",
     ],
 
 }