fix library not found for -lrt on MAC (#7119)

paddle/pybind/CMakeLists.txt

@@ -3,7 +3,9 @@ if(WITH_PYTHON)
     SRCS pybind.cc exception.cc protobuf.cc const_value.cc
     DEPS pybind python backward proto_desc paddle_memory executor prune init
     ${GLOB_OP_LIB})
+  if(NOT APPLE AND NOT ANDROID)
     target_link_libraries(paddle_pybind rt)
+  endif(NOT APPLE AND NOT ANDROID)
 endif(WITH_PYTHON)
 
 if(WITH_DOC)