Fix compilation with os2threads

Signed-off-by: Dave Yeo <daveryeo@telus.net>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

compat/os2threads.h

@@ -32,6 +32,8 @@
 #undef __STRICT_ANSI__          /* for _beginthread() */
 #include <stdlib.h>
 
+#include "libavutil/mem.h"
+
 typedef TID  pthread_t;
 typedef void pthread_attr_t;