meson: don't require libelf for r600 without LLVM

r600 doesn't have a hard requirement on LLVM, and therefore doesn't have
a hard requirement on libelf. Currently the logic doesn't allow that
however.

Distro-bug: https://bugs.gentoo.org/669058


Fixes: 5060c51b
       ("meson: build r600 driver")
Reviewed-by: Matt Turner <mattst88@gmail.com>
(cherry picked from commit 4e785fb3)