[OE-core] having llvm available for gallium

[Trevor Woerner]

I think this might be a casualty of RSS, but when trying to build mesa for the
gizmo2 I get:

	| checking for wayland-scanner... no
	| checking for XCB_DRI2... yes
	| checking for x86_64-oe-linux-llvm-config... no
	| checking for llvm-config... no
	| checking for RADEON... yes
	| configure: error: LLVM is required to build Gallium R300 on x86 and x86_64

There already is a dependency on llvm${MESA_LLVM_RELEASE} so I'm not sure what
magic is required to make llvm-config available.