Fix missing string_view include

Change-Id: I6d40bf17168d22b38e85e045e12228b67caf955e
diff --git a/pw_dumb_io/public/pw_dumb_io/dumb_io.h b/pw_dumb_io/public/pw_dumb_io/dumb_io.h
index b6da8c4..a408557 100644
--- a/pw_dumb_io/public/pw_dumb_io/dumb_io.h
+++ b/pw_dumb_io/public/pw_dumb_io/dumb_io.h
@@ -37,6 +37,7 @@
 
 #include <cstddef>
 #include <cstring>
+#include <string_view>
 
 #include "pw_span/span.h"
 #include "pw_status/status.h"