clang-tidy: ignore making members static

CMake has lots of instances of this which is outside the scope of this
topic right now.

.clang-tidy

@@ -17,6 +17,7 @@ modernize-*,\
 -modernize-use-transparent-functors,\
 performance-*,\
 readability-*,\
+-readability-convert-member-functions-to-static,\
 -readability-function-size,\
 -readability-identifier-naming,\
 -readability-implicit-bool-conversion,\