move inclusion of linux headers for kd.h, soundcard.h, vt.h to bits

maintainer's note: while musl does not use the linux kernel headers,
it does provide these three sys/* headers which do nothing but include
the corresponding linux/* headers, since the sys/* versions are the
ones documented for application use (and they arguably provide
interfaces that are not linux-specific but common to other unices).
these headers should probably not be provided by libc (rather by a
separate package), but as long as they are, use the bits header
framework as an aid to out-of-tree ports of musl for non-linux systems
that want to implement them in some other way.

arch/generic/bits/kd.h

+#include <linux/kd.h>

arch/generic/bits/soundcard.h

+#include <linux/soundcard.h>

arch/generic/bits/vt.h

+#include <linux/vt.h>

include/sys/kd.h

-#include <linux/kd.h>
+#include <bits/kd.h>

include/sys/soundcard.h

-#include <linux/soundcard.h>
+#include <bits/soundcard.h>

include/sys/vt.h

-#include <linux/vt.h>
+#include <bits/vt.h>