install includes in DIST_DIR/include/vpx, move vpx_codec/ to vpx/

This renames the vpx_codec/ directory to vpx/, to allow applications
to more consistently reference these includes with the vpx/ prefix.
This allows the includes to be installed in /usr/local/include/vpx
rather than polluting the system includes directory with an
excessive number of includes.

Change-Id: I7b0652a20543d93f38f421c60b0bbccde4d61b4f

example_xma.c

@@ -15,10 +15,10 @@
 #include <string.h>
 #define VPX_CODEC_DISABLE_COMPAT 1
 #include "vpx_config.h"
-#include "vpx_decoder.h"
-#include "vpx_integer.h"
+#include "vpx/vpx_decoder.h"
+#include "vpx/vpx_integer.h"
 #if CONFIG_VP8_DECODER
-#include "vp8dx.h"
+#include "vpx/vp8dx.h"
 #endif
 
 static char *exec_name;