alsa-utils: disable manpages generation from reStructured text

alsa-utils: disable manpages generation from reStructured text

The alsaucm man page rst source file is missing in the tarball. When rst2man
is detected on the host, build fails:

make[2]: *** No rule to make target 'alsaucm.1', needed by 'all-am'.  Stop.

Upstream added[1] the missing file to the tarball to fix this issue. But since
we don't need the manpage to begin with, just disable rst2man to shorten build
time by a few milliseconds.

[1] http://git.alsa-project.org/?p=alsa-utils.git;a=commitdiff;h=c6bdde171e1532f7b37333a5a746b6e662f12c53

Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Acked-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>