Revert "build: use allowed over enabled"

The meson options used here are dependency checks which should be
handled by the disabler flow.

This reverts commit dabaffecf4eb345e71ee942a9035ddaa5e0eb8ac.

Change-Id: Ia81d168d975efc2c97fd0fefcfee624f2571e1e8
Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
diff --git a/example/meson.build b/example/meson.build
index 4911a67..c7ebf61 100644
--- a/example/meson.build
+++ b/example/meson.build
@@ -14,7 +14,7 @@
 endif
 
 assert(
-    not get_option('examples').allowed() or has_asio,
+    not get_option('examples').enabled() or has_asio,
     'Boost is required when examples are enabled'
 )