autotools: silence gcc warnings in libtool code

autotools: silence gcc warnings in libtool code

```
./.libs/lt-upload-pausing.c: In function 'lt_fatal':
./.libs/lt-upload-pausing.c:593:1: warning: function might be candidate for attribute 'noreturn' [-Wsuggest-attribute=noreturn]
  593 | lt_fatal (const char *file, int line, const char *message, ...)
      | ^~~~~~~~
```
https://github.com/curl/curl/actions/runs/12611924141/job/35148104431?pr=15911#step:11:264

Ref: https://gcc.gnu.org/onlinedocs/gcc/Warning-Options.html#index-Wsuggest-attribute_003d

Closes #15915