Fix -Wmismatched-tags

Summary: Fix -Wmismatched-tags as glean build got broken after D71528763

Reviewed By: kbojarczuk

Differential Revision: D71616836

fbshipit-source-id: 7fcee4f1a0f6dc10b6d49a507d45f2c58dd25e7f

Author: iamirzhan

glean/rts/ffi.h

@@ -14,8 +14,12 @@
 
 #ifdef __cplusplus
 template <typename T>
+#ifdef __HSC2HS__
+struct HsArray;
+#else
 class HsArray;
 #endif
+#endif
 
 #ifdef __cplusplus
 namespace facebook {