Exclude cpp equality operators from `extern "C"`

Summary:
`extern "C"` disables name mangling, hence input parameter types does not influence the name. That makes it impossible to have several equality operators with `extern "C"` linkage (for different types).

One such operator is defined in Windows SDK, in `guiddef.h`. It in turn is included in `winnt.h` inside `extern "C" { ... }` block. Trying to compile file which both is dependent both on `winnt.h` and `Yoga.h` results in:
```
Yoga.h(50): error C2733: 'operator ==': second C linkage of overloaded function not allowed
guiddef.h(192): note: see declaration of 'operator =='
```

In general it doesn't make much sense to have cpp specific operator to have `extern "C"` linkage, so the change doesn't introduce any controlling flag (mangling on/off).

Note that it's breaking binary compatibility and yoga library should be rebuilt if those operators are used.

Reviewed By: milend

Differential Revision: D10418395

fbshipit-source-id: 2f1cccff26165e638b9a07eece07d94fccfa5e5a

ReactCommon/yoga/yoga/Yoga.h

@@ -47,9 +47,13 @@ extern const YGValue YGValueAuto;
 
 #ifdef __cplusplus
 
+YG_EXTERN_C_END
+
 extern bool operator==(const YGValue& lhs, const YGValue& rhs);
 extern bool operator!=(const YGValue& lhs, const YGValue& rhs);
 
+YG_EXTERN_C_BEGIN
+
 #endif
 
 typedef struct YGConfig* YGConfigRef;