codec/vaapi: return error on unsupported environment

Make it buildable on unsupported environment.
Return error on runtime.

pkg/codec/vaapi/vp8.go

@@ -1,8 +1,9 @@
+// +build dragonfly freebsd linux netbsd openbsd solaris
+
 package vaapi
 
 // reference: https://github.com/intel/libva-utils/blob/master/encode/vp8enc.c
 
-// #cgo pkg-config: libva libva-drm
 // #include <fcntl.h>
 // #include <stdint.h>
 // #include <stdio.h>