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: facebook/yoga#1127

Reviewed By: yungsters

Differential Revision: D40024450

Pulled By: yungsters

fbshipit-source-id: f6f01eadccb13d593c68300059e96f4b0bbc9fb6

ReactCommon/yoga/yoga/Yoga.cpp

@@ -302,7 +302,7 @@ YOGA_EXPORT void YGNodeReset(YGNodeRef node) {
   node->reset();
 }
 
-int32_t YGConfigGetInstanceCount(void) {
+YOGA_EXPORT int32_t YGConfigGetInstanceCount(void) {
   return gConfigInstanceCount;
 }
 
@@ -4324,7 +4324,7 @@ YOGA_EXPORT void YGConfigSetExperimentalFeatureEnabled(
   config->experimentalFeatures[feature] = enabled;
 }
 
-inline bool YGConfigIsExperimentalFeatureEnabled(
+YOGA_EXPORT bool YGConfigIsExperimentalFeatureEnabled(
     const YGConfigRef config,
     const YGExperimentalFeature feature) {
   return config->experimentalFeatures[feature];