libstore: Fix FreeBSD build

FreeBSD was left out of a few refactors over the last few months.

Add a header and register the store implementation so it's back
to working as well as it was before.

Change-Id: I6f7b2ceb557c290f2d9e0d7f207b3fea87b353ed

src/libstore/platform/freebsd.cc

@@ -1,5 +1,6 @@
 #include "platform/freebsd.hh"
 #include "regex.hh"
+#include "strings.hh"
 #include <sys/param.h>
 #include <sys/queue.h>
 #include <sys/socket.h>
@@ -139,4 +140,10 @@ void FreeBSDLocalStore::findPlatformRoots(UncheckedRoots & unchecked)
         }
     }
 }
+
+void registerLocalStore()
+{
+    StoreImplementations::add<FreeBSDLocalStore, LocalStoreConfig>();
+}
+
 }