exprtk: added missing default constructors for clang <= 3.8

src/core/math/extern/exprtk.h

@@ -748,10 +748,10 @@ namespace exprtk
          namespace details
          {
-            struct unknown_type_tag {};
-            struct real_type_tag    {};
-            struct complex_type_tag {};
-            struct int_type_tag     {};
+            struct unknown_type_tag { unknown_type_tag() {} };
+            struct real_type_tag    { real_type_tag() {}    };
+            struct complex_type_tag { complex_type_tag() {} };
+            struct int_type_tag     { int_type_tag() {}     };
             template <typename T>
             struct number_type { typedef unknown_type_tag type; };