net/octeontx: fix build for gcc < 4.6
net/octeontx: fix build for gcc < 4.6

-Ofast option supported starting from gcc4.6 [1], for older versions
using "-O3 -ffast-math" instead.

[1] build error:
  CC octeontx_rxtx.o
  cc1: error: invalid option argument ‘-Ofast’

Fixes: 9e747589bd4c ("net/octeontx: add packet transmit burst function")

Signed-off-by: Ferruh Yigit <ba8734c3c506d2e3557bd4ac8fdaccd25bdb9aa7@intel.com>
Acked-by: Santosh Shukla <23b682afe3504ae4f88c66c3cabfb34f87e9916e@caviumnetworks.com>

john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk