graphics/mesa: Fix port when elfutils is installed

By default when building in a clean env (i.e. poudriere) libelf from base
will be used.
When building with an unclean env and if devel/elfutils is installed build
system will be confused and use libelf headers from ${LOCALBASE}/include but
libelf from base.

Fix this.

Sponsored by: Beckhoff Automation GmbH & Co. KG
PR: 275388