amd: Build aco only if radv is enabled

ACO depends on C++14, but radeonsi/radv with LLVM 8,9 do not. Let us
only require it for RADV, since that is the only user.

Fixes: a70a9987 "radv/aco: Setup alternate path in RADV to support the experimental ACO compiler"
Reviewed-by: Marek Olšák <marek.olsak@amd.com>

src/amd/meson.build

@@ -22,7 +22,7 @@ inc_amd = include_directories('.')
 
 subdir('addrlib')
 subdir('common')
-subdir('compiler')
 if with_amd_vk
+  subdir('compiler')
   subdir('vulkan')
 endif