fix missing dll exports (#1127)
fix missing dll exports (#1127)

Summary:
When I use libyogacore.so in other programming languages, it crash with message as

> Could not obtain symbol from the library: dlsym(0x20b84d220, YGConfigIsExperimentalFeatureEnabled): symbol not found

This function is defined as `WIN_EXPORT bool YGConfigIsExperimentalFeatureEnabled` in yoga.h, but is not defined using `YOGA_EXPORT` in yoga.cpp.

X-link: https://github.com/facebook/yoga/pull/1127

Reviewed By: yungsters

Differential Revision: D40024450

Pulled By: yungsters

fbshipit-source-id: f6f01eadccb13d593c68300059e96f4b0bbc9fb6

javache/react-native,javache/react-native,javache/react-native,javache/react-native,facebook/react-native,facebook/react-native,facebook/react-native,facebook/react-native,javache/react-native,javache/react-native,facebook/react-native,javache/react-native,javache/react-native,facebook/react-native,facebook/react-native,facebook/react-native,facebook/react-native,javache/react-native