Fix <net/route.h>.

Previously it couldn't be included alone. Also add a header and include guard,
and fix its siblings' include guards.

Bug: https://code.google.com/p/android/issues/detail?id=82638
Change-Id: I32139a4bda0a6b015508a2319bf4e1a1453345f2
Merge "Fix <net/route.h>."