Wrap C++ headers in `#ifdef __cplusplus`.

The upb libraries can also be accessed from Kotlin Native code, which
understands only C headers, not C++. By adding these `#ifdef` directives, the
C++ headers will appear to be empty in that case.

PiperOrigin-RevId: 599593286

upb/base/status.hpp

@@ -8,6 +8,8 @@
 #ifndef UPB_BASE_STATUS_HPP_
 #define UPB_BASE_STATUS_HPP_
 
+#ifdef __cplusplus
+
 #include "upb/base/status.h"
 
 namespace upb {
@@ -47,4 +49,6 @@ class Status final {
 
 }  // namespace upb
 
+#endif  // __cplusplus
+
 #endif  // UPB_BASE_STATUS_HPP_

upb/mem/arena.hpp

@@ -8,6 +8,8 @@
 #ifndef UPB_MEM_ARENA_HPP_
 #define UPB_MEM_ARENA_HPP_
 
+#ifdef __cplusplus
+
 #include <memory>
 
 #include "upb/mem/arena.h"
@@ -51,4 +53,6 @@ class InlinedArena : public Arena {
 
 }  // namespace upb
 
+#endif  // __cplusplus
+
 #endif  // UPB_MEM_ARENA_HPP_