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>
185 jobs for !16808 with fix-clc-llvm15 in 67 minutes and 15 seconds (queued for 16 seconds)
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