swr: bump minimum supported LLVM version to 6.0

swr: bump minimum supported LLVM version to 6.0

RADV now requires LLVM 6.0 or greater, and thus we can't build dist
tarball because swr requires LLVM 5.0.

Let's bump required LLVM to 6.0 in swr too.

v2: bump also in meson.build (Eric)

Fixes: fd1121e839 ("amd: remove support for LLVM 5.0")
Cc: Tim Rowley <timothy.o.rowley@intel.com>
Cc: Emil Velikov <emil.velikov@collabora.com>
Cc: Dylan Baker <dylan@pnwbakers.com>
Cc: Eric Engestrom <eric.engestrom@intel.com>
Reviewed-by: Eric Engestrom <eric.engestrom@intel.com>
Reviewed-by: Bruce Cherniak <bruce.cherniak@intel.com>