examples/l3fwd: add include for macro definition

examples/l3fwd: add include for macro definition

The header files "l3fwd_em.h" and "l3fwd_em_sequential.h" use the
"__rte_always_inline" macro but don't directly include "rte_common.h" to
get the definition of it. This inclusion is not necessary for
compilation, but the lack of it can confuse some indexers - such as
those in eclipse, which reports the lines:

"static __rte_always_inline uint16_t"

as possible definitions of a variable called "uint16_t". This confusion
leads to uint16_t being flagged as an unknown type in all other parts of
the project being indexed, e.g. across all of DPDK code.

Adding in the include of rte_common.h makes it clear to the indexer that
those lines are  part of a function definition, and that allows eclipse
to correctly recognise uint16_t as a type from stdint.h

Signed-off-by: Bruce Richardson <bruce.richardson@intel.com>