staging: bcm2835-audio: remove incorrect include path

The driver introduces a new instance of the missing-include-dirs warning that
is enabled at the "make W=1" level but has no other output:

cc1: error: /git/arm-soc/drivers/staging/vc04_services/interface/vcos/linuxkernel: No such file or directory [-Werror=missing-include-dirs]

In order to let us enable the warning by default in the future, we should
just remove the incorrect argument here.

Fixes: 23b028c8 ("staging: bcm2835-audio: initial staging submission")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>