libc: Parenthesise qualifier-preserving function definitions

[fuz]

What is this change required for?

[kfv]

This is required to prevent function-like macros with the same name from being expanded in the definitions once they become active in a later C mode. Without the parentheses, the declarator is rewritten and will not compile. This style is used for similar cases such as mempcpy() too.

[kfv]

I wasn't planning to include a description in the commit body, but perhaps a brief explanation like the one above would be useful?

[fuz]

Yes, please include this explanation in the commit body. It appears somewhat puzzling otherwise.
LGTM and approved for commit.