Add missing inclusion of <iomanip>.

This was previously included transitively by GTest, but upstream
no longer includes it.

Bug: 271622675
Test: presubmit
Change-Id: Ie9875348d16ae42a2acaf2ae585cfcffec522c85
diff --git a/src/protozero/test/fake_scattered_buffer.cc b/src/protozero/test/fake_scattered_buffer.cc
index 7b65b29..1c36063 100644
--- a/src/protozero/test/fake_scattered_buffer.cc
+++ b/src/protozero/test/fake_scattered_buffer.cc
@@ -16,6 +16,7 @@
 
 #include "src/protozero/test/fake_scattered_buffer.h"
 
+#include <iomanip>
 #include <sstream>
 #include <utility>