fix srsUE build without RF driver

CMakeLists.txt

@@ -293,8 +293,12 @@ include_directories(${PROJECT_SOURCE_DIR}/lib/include/srslte)
 ########################################################################
 add_subdirectory(lib)
 if(NOT DISABLE_SRSUE)
+  if(RF_FOUND)
     message(STATUS "Building with srsUE")
     add_subdirectory(srsue)
+  else(RF_FOUND)
+    message(STATUS "Building without srsUE due to missing RF driver")
+  endif(RF_FOUND)
 else(NOT DISABLE_SRSUE)
   message(STATUS "Building without srsUE")
 endif(NOT DISABLE_SRSUE)