hwcontext_vaapi: Always include DRM hwcontext header

Fixes building with VAAPI but not libdrm, which was broken by
389f4c3e.  Just unconditionally include
the header, since it doesn't depend on libdrm being present.

libavutil/hwcontext_vaapi.c

@@ -40,15 +40,13 @@
 #include "buffer.h"
 #include "common.h"
 #include "hwcontext.h"
+#include "hwcontext_drm.h"
 #include "hwcontext_internal.h"
 #include "hwcontext_vaapi.h"
 #include "mem.h"
 #include "pixdesc.h"
 #include "pixfmt.h"
 
-#if CONFIG_LIBDRM
-#   include "hwcontext_drm.h"
-#endif
 
 typedef struct VAAPIDevicePriv {
 #if HAVE_VAAPI_X11