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/libutil/file-system.cc

@@ -23,6 +23,8 @@
 # include <io.h>
 #endif
 
+#include "strings-inline.hh"
+
 namespace fs = std::filesystem;
 
 namespace nix {