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
diff --git a/vp8/common/onyx.h b/vp8/common/onyx.h
index b66c400..4287219 100644
--- a/vp8/common/onyx.h
+++ b/vp8/common/onyx.h
@@ -16,7 +16,7 @@
 {
 #endif
 
-#include "vpx_codec/internal/vpx_codec_internal.h"
+#include "vpx/internal/vpx_codec_internal.h"
 #include "vpx_scale/yv12config.h"
 #include "type_aliases.h"
 #include "ppflags.h"