package/mesa3d: add dependency on elfutils for R600 with LLVM

package/mesa3d: add dependency on elfutils for R600 with LLVM

Gallium R600 needs libelf when mesa is built with llvm support.
Because of this, toolchain must use either uClibc or glibc, as
musl is not currently compatible with elfutils.

This patch solves:
configure: error: r600 requires libelf when using llvm

Fixes:

  http://autobuild.buildroot.org/results/8845ff0f28d3273ebe884126b85cd7c4a905d81b/

Signed-off-by: Valentin Korenblit <valentin.korenblit@smile.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>