libstore: Move State to an anonymous namespace

Having a State class in the nix namespace is asking
for ODR trouble. This class is already private to the
translation unit, let's move it into an anonymous namespace.

src/libstore/builtins/buildenv.cc

@@ -10,12 +10,16 @@
 
 namespace nix {
 
+namespace {
+
 struct State
 {
     std::map<Path, int> priorities;
     unsigned long symlinks = 0;
 };
 
+} // namespace
+
 /* For each activated package, create symlinks */
 static void createLinks(State & state, const Path & srcDir, const Path & dstDir, int priority)
 {