route: remove unnecessary include of private linux/if.h

route: remove unnecessary include of private linux/if.h

Including linux/if.h in netlink/route/link.h causes issues
in cases where libnl is used in conjuntion with other third
party libraries that include net/if.h. Seems to be a long
checkered history of symbol collisions between these two
files. As it turns out, including linux/if.h from within
netlink/route/link.h is actually unecessary. I resurrected
a forgotten path from this thread:

    http://lists.infradead.org/pipermail/libnl/2012-April/000525.html

By removing the include as the patch suggests we can get
around the nuissance of the symbol collisions.

https://github.com/thom311/libnl/pull/73

Note: This patch is backported from
http://git.infradead.org/users/tgr/libnl.git/patch/50a76998ac36ace3716d3c979b352fac73cfc80a

Change-Id: I6651c84e55870963825180c2732de581e6bb0ab6
Signed-off-by: Nishant Chaprana <n.chaprana@samsung.com>