[unit-test] add std includes for external catch2

include/mapnik/sql_utils.hpp

@@ -37,6 +37,7 @@ MAPNIK_DISABLE_WARNING_POP
 #include <iosfwd>
 #include <regex>
 #include <string>
+#include <sstream>
 
 namespace mapnik { namespace sql_utils {
 

test/unit/datasource/spatial_index.cpp

@@ -22,6 +22,8 @@
 
 #include "catch.hpp"
 
+#include <sstream>
+
 #include <mapnik/quad_tree.hpp>
 #include <mapnik/util/spatial_index.hpp>
 

test/unit/imaging/webp_io.cpp

@@ -3,6 +3,8 @@
 
 #include "catch.hpp"
 
+#include <sstream>
+
 #include <mapnik/image_view.hpp>
 #include <mapnik/webp_io.hpp>