smc_diag.h: fix include from userland

This patch prepares the uapi export by fixing the following error:

.../linux/smc_diag.h:6:27: fatal error: rdma/ib_verbs.h: No such file or directory
 #include <rdma/ib_verbs.h>

Signed-off-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>