Ensure all headers have #pragma once and are in API docs

`///@file` makes them show up in the internal API dos. A tiny few were
missing `#pragma once`.

src/libstore/build/derivation-goal.hh

@@ -1,4 +1,5 @@
 #pragma once
+///@file
 
 #include "parsed-derivations.hh"
 #include "lock.hh"

src/libstore/build/drv-output-substitution-goal.hh

@@ -1,4 +1,5 @@
 #pragma once
+///@file
 
 #include "store-api.hh"
 #include "goal.hh"

src/libstore/build/goal.hh

@@ -1,4 +1,5 @@
 #pragma once
+///@file
 
 #include "types.hh"
 #include "store-api.hh"

src/libstore/build/hook-instance.hh

@@ -1,4 +1,5 @@
 #pragma once
+///@file
 
 #include "logging.hh"
 #include "serialise.hh"

src/libstore/build/local-derivation-goal.hh

@@ -1,4 +1,5 @@
 #pragma once
+///@file
 
 #include "derivation-goal.hh"
 #include "local-store.hh"

src/libstore/build/personality.hh

@@ -1,4 +1,5 @@
 #pragma once
+///@file
 
 #include <string>
 

src/libstore/build/substitution-goal.hh

@@ -1,4 +1,5 @@
 #pragma once
+///@file
 
 #include "lock.hh"
 #include "store-api.hh"

src/libstore/build/worker.hh

@@ -1,4 +1,5 @@
 #pragma once
+///@file
 
 #include "types.hh"
 #include "lock.hh"