build-sys: add a "pixman" feature

For now, pixman is mandatory, but we set config_host.h and Kconfig.
Once compilation is fixed, "pixman" will become actually optional.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
diff --git a/meson_options.txt b/meson_options.txt
index 5c212fc..c9baeda 100644
--- a/meson_options.txt
+++ b/meson_options.txt
@@ -226,6 +226,8 @@
        description: 'l2tpv3 network backend support')
 option('netmap', type : 'feature', value : 'auto',
        description: 'netmap network backend support')
+option('pixman', type : 'feature', value : 'auto',
+       description: 'pixman support')
 option('slirp', type: 'feature', value: 'auto',
        description: 'libslirp user mode network backend support')
 option('vde', type : 'feature', value : 'auto',