Fix bug re: return type of f(un)lockfile wrappers for Windows

Just a tiny bugfix I spotted: The `f(un)lockfile` wrappers in
`format-inl.h` that wrap Windows's `_(un)lock_file` methods are
defined with a trailing return type derived by using decltype on a
hypothetical call to the underlying functions.

The wrappers don't contain a `return` in their bodies, however, so if
the return type of the underlying functions were to ever change from
`void`, there would be a compile error.

IIRC, the ability to write
`return [other function returning void]();` in a function returning
void is a new-ish addition to the standard (no idea when). Since these
functions aren't public-facing, and no library code ever attempts to
use their return values, I opted to just declare them `void` instead
of adding `return`s.

Author: mvastola

include/fmt/format-inl.h

@@ -1476,10 +1476,10 @@ template <typename T> struct span {
   size_t size;
 };
 
-template <typename F> auto flockfile(F* f) -> decltype(_lock_file(f)) {
+template <typename F> void flockfile(F* f) {
   _lock_file(f);
 }
-template <typename F> auto funlockfile(F* f) -> decltype(_unlock_file(f)) {
+template <typename F> void funlockfile(F* f) {
   _unlock_file(f);
 }