gtp: #define _UAPI_LINUX_GTP_H_ and not _UAPI_LINUX_GTP_H__

Fix clang build warning:

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

fix by defining  _UAPI_LINUX_GTP_H_ and not _UAPI_LINUX_GTP_H__

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Acked-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: David S. Miller <davem@davemloft.net>