parser: fix clang-7.0.1 warnings

Without this commit clang++-7.0.1 whines

|   CXX      parser.lo
| parser.cc:138:15: warning: braces around scalar initializer [-Wbraced-scalar-init]
|         return Tree({{Node::Type::Range},
|                      ^~~~~~~~~~~~~~~~~~~

lib/parser/parser.cc

@@ -135,7 +135,7 @@ range (const ProcIface::FieldInfoVec& fields, const std::string& lower,
 	if (prange.lower > prange.upper)
 		prange = proc->process_range (field.field, upper, lower);
 
-	return Tree({{Node::Type::Range},
+	return Tree({Node::Type::Range,
 			     std::make_unique<Range>(field.field, field.prefix, field.id,
 						     prange.lower, prange.upper)});
 }