Fix compatibility to ZeroMQ 2.x

This patch fixes this compile problem of building jZMQ for ZeroMQ 2.x:

```sh
libtool: compile:  g++ -DHAVE_CONFIG_H -I. -D_REENTRANT -D_THREAD_SAFE -I/home/ok/tmp/include -I/usr/lib/jvm/jdk1.7.0_21/include -I/usr/lib/jvm/jdk1.7.0_21/include/linux -Wall -g -O2 -MT libjzmq_la-Socket.lo -MD -MP -MF .deps/libjzmq_la-Socket.Tpo -c Socket.cpp  -fPIC -DPIC -o .libs/libjzmq_la-Socket.o
Socket.cpp: In function 'jint Java_org_zeromq_ZMQ_00024Socket_sendByteBuffer(JNIEnv*, jobject, jobject, jint)':
Socket.cpp:576:44: error: cannot convert 'jbyte* {aka signed char*}' to 'zmq_msg_t*' for argument '2' to 'int zmq_send(void*, zmq_msg_t*, int)'
Socket.cpp: In function	'jint Java_org_zeromq_ZMQ_00024Socket_recvByteBuffer(JNIEnv*, jobject, jobject, jint)':
Socket.cpp:708:43: error: cannot convert 'jbyte* {aka signed char*}' to 'zmq_msg_t*' for argument '2' to 'int zmq_recv(void*, zmq_msg_t*, int)'
Socket.cpp: At global scope:
Socket.cpp:507:10: warning: 'jboolean s_zerocopy_init(JNIEnv*, zmq_msg_t*, jobject, jint)' defined but not used [-Wunused-function]
```