libyang: support returning binary data

Sorry, no test :), but I tested this via the Python bindings along with
user RPCs.

Change-Id: I3f81d41c5142f1ee0a22aa24eef5a034ade927fa
diff --git a/src/libyang_utils.cpp b/src/libyang_utils.cpp
index 0885c64..e6b825d 100644
--- a/src/libyang_utils.cpp
+++ b/src/libyang_utils.cpp
@@ -26,6 +26,8 @@
         return std::string(value->string());
     case LY_TYPE_ENUM:
         return enum_{std::string(value->enm()->name())};
+    case LY_TYPE_BINARY:
+        return std::string{value->binary()};
     default: // TODO: implement all types
         return "(can't print)"s;
     }