Fix an apparent mis-merge that happened in r274082. Before that, on the

Fix an apparent mis-merge that happened in r274082.  Before that, on the
10-stable branch, this makefile had WARNS=2, and on head the value is
still 2, but in the MFC done in r274082 it got changed to 3, causing build
failures when building with gcc.  This direct commit to 10 goes back to
WARNS=2.

git-svn-id: svn://svn.freebsd.org/base/stable/10@274323 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f