Fix test on compilers without char32_t

As the include for BOOST_NO_CXX11_CHAR32_T was missing, the test didn't compile.

test/concepts/test_bug_11988.cpp

@@ -9,6 +9,7 @@
 *
 */
 
+#include <boost/config.hpp>
 
 #ifndef BOOST_NO_CXX11_CHAR32_T
 
@@ -111,4 +112,4 @@ int main()
 
 int main() { return 0; }
 
-#endif
+#endif