Update cmake/FindFreeRTOS.cmake

Done

Co-authored-by: Julien Staub <atsju2@yahoo.fr>

cmake/FindFreeRTOS.cmake

@@ -19,7 +19,7 @@ find_path(FreeRTOS_COMMON_INCLUDE
     NO_DEFAULT_PATH
 )
 
-if(FreeRTOS_COMMON_INCLUDE MATCHES "FreeRTOS_COMMON_INCLUDE-NOTFOUND")
+if(NOT FreeRTOS_COMMON_INCLUDE)
     message(WARNING "FreeRTOS common include path not found, build might fail")
 endif()