examples/l3fwd: fix compilation with clang 3.5
examples/l3fwd: fix compilation with clang 3.5

Fix the following error:
  error: unused function 'l3fwd_simple_forward'

The l3fwd_simple_forward() is maybe unused, due to compilation options
(APP_LOOKUP_METHOD, ENABLE_MULTI_BUFFER_OPTIMIZE). As the combinatorial
is quite big, it looks simpler to add the __attribute__((unused)) on
this function, so that the compiler does not complain.

Signed-off-by: Olivier Matz <dfcc1510895413197abb4336e39bb0a3906cba71@6wind.com>

phermansson/dpdk,phermansson/dpdk,phermansson/dpdk,phermansson/dpdk