gtp: #define #define _GTP_H_ and not #define _GTP_H

Fix clang build warning:

./include/net/gtp.h:1:9: warning: '_GTP_H_' is used as a header
guard here, followed by #define of a different macro [-Wheader-guard]

fix by defining _GTP_H_ and not _GTP_H

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: David S. Miller <davem@davemloft.net>