libfec: pack the fec_header structure

libfec: pack the fec_header structure

The fec_header structure is generated build time and stored on disk.
The fec_header might be build on a 64 bits machine while it is read
per a 32 bits device or the other way around.  In such situations, the
fec_header fields are not aligned as expected by the device and it
fails to read the fec_header structure.

This patch makes the fec_header packed.

Change-Id: I77546f850cea5559909971a1192e1a7616d22d50
Signed-off-by: Jeremy Compostella <jeremy.compostella@intel.com>