refactor: unit_text.h header include moved to text-related targets

example/alternative_namespaces/units_str.h

@@ -1,7 +1,9 @@
 #pragma once
 
+#include <units/bits/unit_text.h>
 #include <units/isq/si/prefixes.h>
 #include <units/quantity.h>
+
 // get at the units text of the quantity, without its numeric value
 inline auto constexpr units_str(const units::Quantity auto& q)
 {