src: Allow for stdplus to be vendored

Change-Id: I0ade83798e1458e31b011119f95fc17c30617153
Signed-off-by: William A. Kennington III <wak@google.com>
diff --git a/src/meson.build b/src/meson.build
index cb076f5..5a485e6 100644
--- a/src/meson.build
+++ b/src/meson.build
@@ -1,6 +1,6 @@
 sdeventplus_deps = [
   dependency('libsystemd', version: '>=239'),
-  dependency('stdplus'),
+  dependency('stdplus', fallback: ['stdplus', 'stdplus']),
 ]
 
 sdeventplus_headers = include_directories('.')