clc: Fix build with llvm-15.

opencl_c_h is defined only for llvm < 15.

Fixes: bcc2df48 ("clc: speed up compilation by not relying on opencl-c.h")
Signed-off-by: Vinson Lee <vlee@freedesktop.org>
Reviewed-by: Jesse Natalie <jenatali@microsoft.com>
Reviewed-by: Karol Herbst <kherbst@redhat.com>
Part-of: <!16808>