Avoid symbol redefinition

Apprently clang and msvc are fine with it, but GCC isn't.

Author: lniccoli

src/internal/sio_client_impl.h

@@ -89,7 +89,7 @@ void set_##__FIELD__(__TYPE__ const& l) \
         // Client Functions - such as send, etc.
         void connect(const std::string& uri);
 
-        socket::ptr const& socket(const std::string& nsp);
+        ::sio::socket::ptr const& socket(const std::string& nsp);
 
         // Closes the connection
         void close();

src/sio_client.h

@@ -62,7 +62,7 @@ namespace sio
 
         void set_reconnect_delay_max(unsigned millis);
 
-        socket::ptr const& socket(const std::string& nsp = "");
+        ::sio::socket::ptr const& socket(const std::string& nsp = "");
 
         // Closes the connection
         void close();