Better-scope Store forward declarations

(cherry picked from commit 685395332d)

src/libstore/derivations.hh

@@ -13,6 +13,7 @@
 
 namespace nix {
 
+class Store;
 
 /* Abstract syntax of derivations. */
 

src/libstore/path.hh

@@ -5,7 +5,6 @@
 
 namespace nix {
 
-class Store;
 struct Hash;
 
 class StorePath

src/libstore/realisation.hh

@@ -7,6 +7,8 @@
 
 namespace nix {
 
+class Store;
+
 struct DrvOutput {
     // The hash modulo of the derivation
     Hash drvHash;