fix includes

Author: scotthart

google/cloud/spanner/uuid.cc

@@ -15,6 +15,7 @@
 #include "google/cloud/spanner/uuid.h"
 #include "google/cloud/internal/make_status.h"
 #include "absl/container/flat_hash_map.h"
+#include "absl/strings/str_format.h"
 #include "absl/strings/strip.h"
 
 namespace google {