使用cocoapod加入第三方库，打开项目有时会出现编译错误，错误如下
Undefined symbols for architecture x86_64:
"_OBJC_CLASS_$_AFHTTPRequestOperationManager", referenced from:
objc-class-ref in AFNetworkingUtil.o
ld: symbol(s) not found for architecture x86_64
clang: error: linker command failed with exit code 1 (use -v to see invocation)Undefi
