cloud/agent/api: add missing placeholder

Change-Id: Ia21dc413264d1b4d772869d6db1aff25e2a8e586
Reviewed-on: https://review.monogon.dev/c/monogon/+/1106
Tested-by: Jenkins CI
Reviewed-by: Mateusz Zalega <mateusz@monogon.tech>
diff --git a/cloud/agent/api/gomod-generated-placeholder.go b/cloud/agent/api/gomod-generated-placeholder.go
new file mode 100644
index 0000000..778f64e
--- /dev/null
+++ b/cloud/agent/api/gomod-generated-placeholder.go
@@ -0,0 +1 @@
+package api