Optional: Move optional-lite to the include/third-party directory

This fixes the use case of adding the include folder
manually to an outside project.

include/icsneo/third-party/optional-lite/BUCK

@@ -0,0 +1,11 @@
+prebuilt_cxx_library(
+  name = 'optional-lite',
+  header_namespace = '',
+  header_only = True,
+  exported_headers = subdir_glob([
+    ('include/nonstd', '**/*.hpp'),
+  ]),
+  visibility = [
+    'PUBLIC',
+  ],
+)