tinyalsa: Fix overwrite of asound.h

As stated by Maarten ter Huurne on the mailing list, the install of the
asound.h headers overwrite the one from the toolchains or kernel
headers.
Since tinyals's header is only a subset of the 'real' asound.h, there is
no need to overwrite it.

Signed-off-by: Maxime Hadjinlian <maxime.hadjinlian@gmail.com>
Cc: Maarten ter Huurne <maarten@treewalker.org>
Cc: Thomas Petazzoni  <thomas.petazzoni@free-electrons.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>