net: net_pkt: move #ifdef to before doxygen block

This moves the #ifdef to enclose the associated doxygen block.
Or else the function prototype are ignored/skipped but doxygen
still processes the block, and it will apply the documentation
to the next function in the file... and it will complain about
parameters not being found.

Signed-off-by: Daniel Leung <daniel.leung@intel.com>