Add an option to allow the user to disable examples, this will allow to
build pipewire without alsa. Without this option, build with
-Dpipewire-alsa=false -Dalsa=false fails on:
src/examples/meson.build:47:0: ERROR: Unknown variable "alsa_dep".
Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>