fix(OpenThread): fixes file list in CMakeLists.txt

Author: SuGlider

CMakeLists.txt

@@ -161,7 +161,7 @@ set(ARDUINO_LIBRARY_NetBIOS_SRCS libraries/NetBIOS/src/NetBIOS.cpp)
 
 set(ARDUINO_LIBRARY_OpenThread_SRCS
   libraries/OpenThread/src/OThreadCLI.cpp
-  libraries/OpenThread/src/OThreadCLI_Util.c)
+  libraries/OpenThread/src/OThreadCLI_Util.cpp)
 
   set(ARDUINO_LIBRARY_PPP_SRCS
   libraries/PPP/src/PPP.cpp