Fix missing prototype error, void end of function error on Windows.

MinGW64 gcc silently ignores C prototype with no arguments, issuing
a missing prototype error instead. set_platform_locale must return
a free-able char* even if all of LC_ALL, LC_MESSAGES, and LANG are
unset.

gnucash/gnucash-locale-platform.h

@@ -25,6 +25,6 @@
 #ifndef GNUCASH_LOCALE_PLATFORM_H
 #define GNUCASH_LOCALE_PLATFORM_H
 
-char *set_platform_locale();
+char *set_platform_locale(void);
 
 #endif

gnucash/gnucash-locale-windows.c

@@ -33,7 +33,7 @@
  * retrieve the Windows locale and set POSIX to match.
  */
 char *
-set_platform_locale()
+set_platform_locale(void)
 {
     WCHAR lpLocaleName[LOCALE_NAME_MAX_LENGTH];
     char *locale = NULL;
@@ -68,4 +68,5 @@ set_platform_locale()
         setlocale (LC_ALL, locale);
         return locale;
     }
+    return g_strdup("C");
 }