Fixup ppm_tests missing include

BUG=b:319124643
TEST=make local_tests

Change-Id: Id0ebba8b0deb25cfa2dba149317a3f297f75d8f3
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/third_party/usb/um_ppm/+/5255582
Tested-by: Abhishek Pandit-Subedi <abhishekpandit@google.com>
Reviewed-by: Pavan Holla <pholla@google.com>
diff --git a/test/ppm_tests.cc b/test/ppm_tests.cc
index b431e16..c8342c1 100644
--- a/test/ppm_tests.cc
+++ b/test/ppm_tests.cc
@@ -13,6 +13,7 @@
 #include <deque>
 #include <memory>
 #include <mutex>
+#include <optional>
 #include <thread>
 #include <utility>
 #include <vector>