vlc_common: fix swab() calls on win32 that don't use const on source pointer

(cherry picked from commit a9e0b1124e)
Signed-off-by: Steve Lhomme <robux4@ycbcr.xyz>

include/vlc_common.h

@@ -947,6 +947,11 @@ static inline void SetQWLE (void *p, uint64_t qw)
 #       define O_NONBLOCK 0
 #   endif
 
+/* the mingw32 swab() and win32 _swab() prototypes expect a char* instead of a
+   const void* */
+#  define swab(a,b,c)  swab((char*) (a), (char*) (b), (c))
+
+
 #   include <tchar.h>
 #endif /* _WIN32 */