libctf: use __attribute__((__gnu_printf__)) where appropriate

libctf: use __attribute__((__gnu_printf__)) where appropriate

We don't use any GNU-specific printf args, but this prevents warnings about
%z, observed on MinGW even though every libc anyone is likely to use there
supports %z perfectly well, and we're not stopping using it just because
MinGW complains.  Doing this means we stand more chance of seeing *actual*
problems on such platforms without them being drowned in noise.

We turn this off on clang, which doesn't support __gnu_printf__.

Suggested by Eli Zaretskii.

libctf/
	PR libctf/31863
	* ctf-impl.h (_libctf_printflike_): Use __gnu_printf__.