concatStrings: Give compiler access to definition for inlining

... at call sites that are may be in the hot path.

I do not know how clever the compiler gets at these sites.
My primary concern is to not regress performance and I am confident
that this achieves it the easy way.

src/libstore/derivations.cc

@@ -10,6 +10,8 @@
 #include <boost/container/small_vector.hpp>
 #include <nlohmann/json.hpp>
 
+#include "strings-inline.hh"
+
 namespace nix {
 
 std::optional<StorePath> DerivationOutput::path(const StoreDirConfig & store, std::string_view drvName, OutputNameView outputName) const