net/failsafe: fix build on FreeBSD 10 with GCC 4.8

net/failsafe: fix build on FreeBSD 10 with GCC 4.8

ctype.h is not compilable in BSD 10 on GCC 4.8 in C11 mode.

  CC failsafe.o
In file included from /usr/include/_ctype.h:94:0,
                 from /usr/include/ctype.h:46,
                 from /root/dpdk.org/build/include/rte_common.h:50,
                 from /root/dpdk.org/build/include/rte_memory.h:57,
                 from /root/dpdk.org/build/include/rte_malloc.h:45,
                 from /root/dpdk.org/drivers/net/failsafe/failsafe.c:35:
/usr/include/runetype.h:92:22: error: expected '=', ',', ';', 'asm' or
'__attribute__' before 'const'
 extern _Thread_local const _RuneLocale *_ThreadRuneLocale;
                      ^
/usr/include/runetype.h: In function '__getCurrentRuneLocale':
/usr/include/runetype.h:96:6: error: '_ThreadRuneLocale' undeclareds
(first use in this function)
  if (_ThreadRuneLocale)
      ^

The fix is to put GCC in gnu99 mode instead.

Fixes: a46f8d584eb8 ("net/failsafe: add fail-safe PMD")

Signed-off-by: Gaetan Rivet <gaetan.rivet@6wind.com>