blob: Constify name argument of blob_get_def_by_name().

blob: Constify name argument of blob_get_def_by_name().

This function does not modify the string, although the char pointer
is used as the ->data pointer of an osl object, which is non-constant.

We need to cast away the const qualifier to avoid a compiler warning,
but that's still better than accepting only non-constant strings,
as this means to put the cast into the callers.