vlc_memstream: fix include-guard

Simple fix of typo in the include-guard so that it actually does what
it is supposed to do.
Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>

include/vlc_memstream.h

@@ -19,7 +19,7 @@
  *****************************************************************************/
 
 #ifndef VLC_MEMSTREAM_H
-# define VLC_MEMSREAM_H 1
+# define VLC_MEMSTREAM_H 1
 
 # include <stdarg.h>
 # include <stdio.h>