Build mbed-os-example-lora only for lora targets

mbed-os-example-lora should be only compiled for targets which support lora.

(cherry picked from commit b61912c)
Required to pass CI now that LoRaWAN example has removed its own
conditions.

Author: Kimmo Vaisanen

tools/test/examples/examples.json

@@ -236,7 +236,7 @@
         "mbed": [],
         "test-repo-source": "github",
         "features" : [],
-        "targets" : [],
+        "targets" : ["DISCO_L072CZ_LRWAN1", "MTB_MTS_XDOT", "MTS_MDOT_F411RE"],
         "toolchains" : [],
         "exporters": [],
         "compile" : true,