Remove duplicate declaraton of the if_inc_counter() function after r272244.
if_var.h has the expected on and if_var.h include ifq.h and thus we get
duplicates.  It seems only one cavium ethernet file actually includes ifq.h
directly which might be another cleanup to be done but need to test first.


git-svn-id: svn+ssh://svn.freebsd.org/base/head@272260 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f