win32: fix semantics of POSIX 2008 locale stubs

This sliences some warnings about unused values and statements with no
effect, but it also fixes a logic error with freelocale(), since
previously it would not work as expected when used in the body of an if
statement without braces.

Uses real functions, because with macros, I don't think there is a way
to silence the "statement with no effect" warnings in the case where the
return value of uselocale() is ignored.

As for replacing the these functions with working implementations, I
don't think this is possible for mpv's use-case, since MSVCRT does not
support UTF-8 locales, or any locale with multibyte characters that are
three or more bytes long. See:
https://docs.microsoft.com/en-us/cpp/c-runtime-library/reference/setlocale-wsetlocale

osdep/io.c

@@ -799,4 +799,18 @@ int msync(void *addr, size_t length, int flags)
 }
 #endif
 
+locale_t newlocale(int category, const char *locale, locale_t base)
+{
+    return (locale_t)1;
+}
+
+locale_t uselocale(locale_t locobj)
+{
+    return (locale_t)1;
+}
+
+void freelocale(locale_t locobj)
+{
+}
+
 #endif // __MINGW32__

osdep/io.h

@@ -193,11 +193,12 @@ int msync(void *addr, size_t length, int flags);
 #define glob(...) mp_glob(__VA_ARGS__)
 #define globfree(...) mp_globfree(__VA_ARGS__)
 
-// There is not anything that helps with this on Windows.
+// These are stubs since there is not anything that helps with this on Windows.
 #define locale_t int
-#define newlocale(a, b, c) 1
-#define uselocale(a) 1
-#define freelocale(a)
+#define LC_ALL_MASK 0
+locale_t newlocale(int, const char *, locale_t);
+locale_t uselocale(locale_t);
+void freelocale(locale_t);
 
 #else /* __MINGW32__ */