[libnetmap] Fix 32 bit compilation under gcc-6.4

Use uintptr_t to cast a uint64_t to a pointer type.
Yeah, it isn't technically correct for platforms with pointers

64 bits, but it's fine here.

This fixes 32 bit compat library builds on amd64 and also
mips32 builds.

Reviewed by: imp
Differential Revision: https://reviews.freebsd.org/D26790