Don't use store-api.hh in worker-protocol.hh

Using abstract types like can help cut down on compilation time, both
from scratch, and especially incremental builds during development. The
idea is that `worker-protocol.hh` can declare all the (de)serializers, but
only again abstract types; when code needs to use some (de)serializers, it can
include headers just for the data types it needs to (de)serialize.

`store-api.hh` in particular is a bit of a sledgehammer, and the data
types we want to serialize have their own headers.

src/libstore/path-info.cc

@@ -1,5 +1,6 @@
 #include "path-info.hh"
 #include "worker-protocol.hh"
+#include "store-api.hh"
 
 namespace nix {