Clang's missing-variable-declarations is useful to set as an error
when variables aren't declared as static when they should be.
C++ support for RNA files [0] caused clang to print many
missing-variable-declarations warnings (which I set to error locally).
This change suppresses the warning for RNA.
This should be resolved but for now suppress the RNA warning so this
can be treated as an error elsewhere.
[0]: d154ebfa83
a4583f1cb8