Add cppcoreguidelines-pro-type-cstyle-cast to clang-tidy config

Prevents that someone re-introduces C-style casts.
diff --git a/.clang-tidy b/.clang-tidy
index e0afd47..357f046 100644
--- a/.clang-tidy
+++ b/.clang-tidy
@@ -6,6 +6,7 @@
   ,readability-redundant-string-cstr,
   ,readability-redundant-string-init,
   ,readability-simplify-boolean-expr,
+  ,cppcoreguidelines-pro-type-cstyle-cast,
 '
 WarningsAsErrors: '
   ,readability-avoid-const-params-in-decls,
@@ -14,4 +15,5 @@
   ,readability-redundant-string-cstr,
   ,readability-redundant-string-init,
   ,readability-simplify-boolean-expr,
+  ,cppcoreguidelines-pro-type-cstyle-cast,
 '