libifconfig now depends on libm due to usage of log10().

[jhb]

Summary

ld.bfd in particular requires -lm to come after libifconfig on the
command line when linking rescue.

Test Plan

• an amd64 buildworld with CROSS_TOOLCHAIN=amd64-gcc6 was failing and is now fixed. This is due to the changes in r364508.

[adrian]

Lol, who did that? :-)

Oh well!

[freqlabs]

In D26258#583657, @adrian wrote:

Lol, who did that? :-)

Oh well!

Guilty...