meson: Fix no-rtti in llvm detection

Because I clearly wasn't thinking and clearly didn't do a good job
testing. Sigh

Fixes: c5a97d658ec19cc02719d7f86c1b0715e3d9ffc4
       ("meson: fix builds against LLVM built without rtti")
Signed-off-by: Dylan Baker <dylan.c.baker@intel.com>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
1 file changed