Drop -I$includedir/alsa from alsa.pc

Drop -I$includedir/alsa from alsa.pc

We used to put the additional include path $includedir/alsa in
pkgconfig just because some applications have included asoundlib.h
like
  #include <asoundlib.h>
although the canonical form should be
  #include <alsa/asoundlib.h>

However, adding this include path is significantly dangerous due to
possible conflicts of file names like version.h.  It's already the
reason to discourage people using alsa.pc for the packages.

In this patch, the additional include path from alsa.pc is dropped
finally.  At the same time, as a rescue plan for the programs
including via <asoundlib.h>, a stub header file is provided in
include/sound/asoundlib.h.  It just includes alsa/asoundlib.h with a
warning to suggest for replacing with alsa/asoundlib.h.
Actually this is the same file as we install into sys/asoundlib.h, so
the whole changes are very minimal here.

Acked-by: Jaroslav Kysela <perex@perex.cz>
Signed-off-by: Takashi Iwai <tiwai@suse.de>