fix compile error with UdpSocket::recvfrom

ixwebsocket/IXUdpSocket.h

@@ -28,7 +28,7 @@ namespace ix
         // Virtual methods
         bool init(const std::string& host, int port, std::string& errMsg);
         ssize_t sendto(const std::string& buffer);
-        ssize_t recvfrom(void* buffer, size_t length);
+        ssize_t recvfrom(char* buffer, size_t length);
 
         void close();