build: Specify native binaries explicitly when building `capnp` package

From `configure --help`:
  --with-external-capnp   use the system capnp binary (or the one specified
                          with $CAPNP) instead of compiling a new one (useful
                          for cross-compiling)

depends/packages/capnp.mk

@@ -8,6 +8,8 @@ $(package)_dependencies=native_$(package)
 
 define $(package)_set_vars :=
 $(package)_config_opts := --with-external-capnp
+$(package)_config_opts += CAPNP="$$(native_capnp_prefixbin)/capnp"
+$(package)_config_opts += CAPNP_CXX="$$(native_capnp_prefixbin)/capnp-c++"
 $(package)_config_opts_android := --disable-shared
 endef