Fix build race in Makefile.inc1 after r274226 by adding lib/libm__L dependency
for gnu/lib/libdialog

Sponsored by: EMC / Isilon Storage Division


git-svn-id: svn+ssh://svn.freebsd.org/base/head@274270 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f