clc: add commment to clc_optional_features to ensure no padding exists

Hopefully this is good enough.

Signed-off-by: Karol Herbst <git@karolherbst.de>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/23532>

src/compiler/clc/clc.h

@@ -49,6 +49,7 @@ enum clc_spirv_version {
    CLC_SPIRV_VERSION_1_4,
 };
 
+// IMPORTANT: this struct is used for hashing and must not contain any padding.
 struct clc_optional_features {
    bool fp16;
    bool fp64;