Add __attribute__((unused)) to __BIONIC_ERROR_FUNCTION_VISIBILITY

This commit adds `__attribute__((unused))` to
`__BIONIC_ERROR_FUNCTION_VISIBILITY`, so that `open()`, `openat()`,
`snprintf()`, and `sprintf()` don't raise `-Werror,-Wunused-function`
when `_FORTIFY_SOURCE` is enabled.

These errors were hidden because the include directories were passed
with `-isystem` (instead of `-I`) and clang did not report
`-Wunused-function` from `-isystem`.

Bug: 119086738
Test: make checkbuild
Change-Id: I0de71efdbacd90c5c6a419fc0368c92e8efdfd63