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. This just adds `return` to each.

Author: mvastola

include/fmt/format-inl.h

@@ -1477,10 +1477,10 @@ template <typename T> struct span {
 };
 
 template <typename F> auto flockfile(F* f) -> decltype(_lock_file(f)) {
-  _lock_file(f);
+  return _lock_file(f);
 }
 template <typename F> auto funlockfile(F* f) -> decltype(_unlock_file(f)) {
-  _unlock_file(f);
+  return _unlock_file(f);
 }
 
 #ifndef getc_unlocked