Include "ws_symbol_export.h" to make sure WS_DLL_PUBLIC is defined.

Wrap the declaration for C++ while we're at it.

Change-Id: Ifcc1b47bab139f5fb8da8c3dd4f20b1ebb99739e
Reviewed-on: https://code.wireshark.org/review/5418
Reviewed-by: Guy Harris <guy@alum.mit.edu>