Use absolute import path for c++.capnp.

This allows {schema,rpc,rpc-twoparty}.capnp to be used without modification when
relocated outside of the Cap'n Proto codebase.

c++/src/capnp/rpc-twoparty.capnp

@@ -71,7 +71,7 @@
 # two-party protocol initially, and then pair applications with an appropriate proxy written in
 # C++, rather than implement other parameterizations of the RPC protocol directly.
 
-using Cxx = import "c++.capnp";
+using Cxx = import "/capnp/c++.capnp";
 $Cxx.namespace("capnp::rpc::twoparty");
 
 enum Side {

c++/src/capnp/rpc.capnp

@@ -111,7 +111,7 @@
 # bother implementing any other vat network protocol -- just use the correct container type and get
 # it for free.
 
-using Cxx = import "c++.capnp";
+using Cxx = import "/capnp/c++.capnp";
 $Cxx.namespace("capnp::rpc");
 
 # ========================================================================================

c++/src/capnp/schema.capnp

@@ -21,7 +21,7 @@
 # (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS
 # SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 
-using Cxx = import "c++.capnp";
+using Cxx = import "/capnp/c++.capnp";
 
 @0xa93fc509624c72d9;
 $Cxx.namespace("capnp::schema");