Fix linking error in render-test (#15900)
```
Undefined symbols for architecture x86_64:
"mbgl::ProxyFileSource::getRequestCount()", referenced from:
TestRunner::runOperations(std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> > const&, TestMetadata&, RunContext&) in runner.cpp.o
"mbgl::ProxyFileSource::isTrackingActive()", referenced from:
TestRunner::runOperations(std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> > const&, TestMetadata&, RunContext&) in runner.cpp.o
"mbgl::ProxyFileSource::setTrackingActive(bool)", referenced from:
TestRunner::runOperations(std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> > const&, TestMetadata&, RunContext&) in runner.cpp.o
TestRunner::run(TestMetadata&) in runner.cpp.o
"mbgl::ProxyFileSource::getTransferredSize()", referenced from:
TestRunner::runOperations(std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> > const&, TestMetadata&, RunContext&) in runner.cpp.o
```
Fixes Debug build on OS X with clang-8