Fix msys2 build error and warnings

[na-na-hi]

Currently, main fails to build on mingw (msys2) due to missing headers:

llama.cpp: In function 'llama_context* llama_init_from_file(const char*, llama_context_params)':
llama.cpp:1675:23: error: 'time' was not declared in this scope; did you mean 'ftime'?
 1675 |         params.seed = time(NULL);
      |                       ^~~~
      |                       ftime

This is solved by including <ctime>.

Also there are several warnings about printf formats:

llama.cpp: In function 'size_t checked_div(size_t, size_t)':
llama.cpp:258:39: warning: unknown conversion type character 'z' in format [-Wformat=]
  258 |         throw format("error dividing %zu / %zu", a, b);
      |                                       ^

As far as I know, this wraning is bogus and appears to be a bug: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=95130
This is solved by using gnu_printf for format attribute when building for mingw.