block_cache: explicitly instantiate template function

The explicit instantiation is required because the function is defined
in the compilation unit only but referenced externally.

Fixes #4108

repos/os/src/server/block_cache/main.cc

@@ -44,6 +44,9 @@ void Driver<POLICY>::Policy::sync(const typename POLICY::Element *e, char *dst)
 	}
 }
 
+/* explicit instantiation for external reference */
+template void Driver<Policy>::Policy::sync(const typename Policy::Element *, char *);
+
 
 struct Main
 {