meson: Error if LLVM doesn't have rtti when building clover

We already do this for nouveau, but it's required for clover too.
diff --git a/meson.build b/meson.build
index 715c966..4654125 100644
--- a/meson.build
+++ b/meson.build
@@ -1253,6 +1253,8 @@
   if dep_llvm.get_configtool_variable('has-rtti') == 'NO'
     if with_gallium_nouveau
       error('The Nouveau driver requires rtti. You either need to turn off nouveau or use an LLVM built with LLVM_ENABLE_RTTI.')
+    elif with_gallium_opencl
+      error('The Clover OpenCL state tracker requires rtti, you need to turn off clover or use an LLVM built with LLVM_ENABLE_RTTI.')
     endif
     cpp_args += '-fno-rtti'
   endif