add include guard

miniupnpd/pcp_msg_struct.h

@@ -29,6 +29,8 @@ CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
 ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
 POSSIBILITY OF SUCH DAMAGE.
  */
+#ifndef PCP_MSG_STRUCT_H_INCLUDED
+#define PCP_MSG_STRUCT_H_INCLUDED
 
 #define PCP_OPCODE_ANNOUNCE   0
 #define PCP_OPCODE_MAP        1
@@ -314,3 +316,4 @@ typedef struct pcp_filter_option {
 #endif
 #define PCP_FILTER_OPTION_SIZE (24)
 
+#endif /* PCP_MSG_STRUCT_H_INCLUDED */