pw_multisink: Extend CMake support

Change-Id: I90fe28cc33d107fba877de3c9e231a778efdece7
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/86582
Pigweed-Auto-Submit: Ewout van Bekkum <ewout@google.com>
Reviewed-by: Armando Montanez <amontanez@google.com>
Commit-Queue: Auto-Submit <auto-submit@pigweed.google.com.iam.gserviceaccount.com>
diff --git a/pw_multisink/multisink_test.cc b/pw_multisink/multisink_test.cc
index 4a6dc6b..3076aea 100644
--- a/pw_multisink/multisink_test.cc
+++ b/pw_multisink/multisink_test.cc
@@ -17,6 +17,7 @@
 #include <array>
 #include <cstdint>
 #include <optional>
+#include <span>
 #include <string_view>
 
 #include "gtest/gtest.h"