meson: add examples option

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>

meson_options.txt

@@ -2,6 +2,10 @@ option('docs',
        description: 'Build documentation',
        type: 'boolean',
        value: false)
+option('examples',
+       description: 'Build examples',
+       type: 'boolean',
+       value: true)
 option('man',
        description: 'Build manpages',
        type: 'boolean',