Disable USE_POSIX_MAP on OS/2

Change-Id: Ib88ab619fa4e1593e85ca325555f2c4648ac9bc7

vpxenc.c

@@ -10,7 +10,7 @@
 
 #include "vpx_config.h"
 
-#if defined(_WIN32) || !CONFIG_OS_SUPPORT
+#if defined(_WIN32) || defined(__OS2__) || !CONFIG_OS_SUPPORT
 #define USE_POSIX_MMAP 0
 #else
 #define USE_POSIX_MMAP 1