[style] Allow the warn_unused_result attribute.
[style] Allow the warn_unused_result attribute.

This attribute is implemented by both GCC and Clang, and is analogous to
the C++17's [[must_use_result]], albeit useable from C.

Allowing use of this attribute will allow us to help DIF callers avoid
forgetting to check errors. Notably, bindgen supports converting them
into `#[must_use]` annotations.

Signed-off-by: Miguel Young de la Sota <71b8e7f4945fd97b98544cf897992af89646547a@google.com>

lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan