Include materialize.hpp in the view builder test

The test calls detail::view::materialize(), which only the single header
pulled in; the multiple-header build (JSON_TEST_USING_MULTIPLE_HEADERS)
failed with "'materialize' is not a member of detail::view".

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

tests/src/unit-json_view_builder.cpp

@@ -11,6 +11,7 @@
 #include <nlohmann/json.hpp>
 #if JSON_TEST_USING_MULTIPLE_HEADERS
     #include <nlohmann/detail/view/builder.hpp>
+    #include <nlohmann/detail/view/materialize.hpp>
     #include <nlohmann/detail/view/string_ref.hpp>
 #else
     #include <nlohmann/json_view.hpp> // the single header contains the internal headers