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/libexpr/eval.cc

@@ -49,6 +49,8 @@
 
 #endif
 
+#include "strings-inline.hh"
+
 using json = nlohmann::json;
 
 namespace nix {

src/libexpr/nixexpr.cc

@@ -7,6 +7,8 @@
 #include <cstdlib>
 #include <sstream>
 
+#include "strings-inline.hh"
+
 namespace nix {
 
 unsigned long Expr::nrExprs = 0;

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

src/libutil/canon-path.cc

@@ -1,6 +1,7 @@
 #include "canon-path.hh"
 #include "util.hh"
 #include "file-path-impl.hh"
+#include "strings-inline.hh"
 
 namespace nix {
 

src/libutil/file-system.cc

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