net: fix build with gcc 4.4.7 and strict aliasing

net: fix build with gcc 4.4.7 and strict aliasing

include/rte_ip.h:161: error: dereferencing pointer ‘u16’
        does break strict-aliasing rules
include/rte_ip.h:157: note: initialized from here
        ...

The root cause is that, compile enable strict aliasing by default,
while in function rte_raw_cksum() try to convert 'const char *'
to 'const uint16_t *'.

This workaround is to solve the compile issue of GCC strict-aliasing (two
different type pointers should not be point to the same memory address).

For GCC 4.4.7 it will definitely occurs if  flags "-fstrict-aliasing"
and "-Wall" used.

Signed-off-by: Michael Qiu <michael.qiu@intel.com>
[Thomas: add workaround comment]
Acked-by: Thomas Monjalon <thomas.monjalon@6wind.com>