Remove the pack/unpack pragma around net/if.h on MacOS, which
was added to fix a bug in MacOS X 10.4.x on 64-bit platforms.
The bug was fixed in Mac OS X 10.5.0 and, sometime in the last
11 years, compilers started emitting warnings about the fact
that the Apple header stomped over the pragma pack settings
from the workaround. We already don't support versions of MacOS
earlier than 10.5, so there's no point in keeping the workaround.
Signed-off-by: Brian Barrett <bbarrett@amazon.com>