fix: missing include file added

Author: mpusz

src/core/include/mp-units/framework/reference.h

@@ -26,6 +26,7 @@
 #include <mp-units/bits/module_macros.h>
 #include <mp-units/compat_macros.h>
 #include <mp-units/framework/quantity_concepts.h>
+#include <mp-units/framework/quantity_spec.h>
 #include <mp-units/framework/quantity_point_concepts.h>
 #include <mp-units/framework/reference_concepts.h>
 #include <mp-units/framework/representation_concepts.h>