feat: add blocklist_var

[d5ng4i]

Input C/C++ Header

extern int should_be_blocked;

Bindgen Invocation

bindgen::Builder::default()
    .header("input.h")
    .blocklist_var("should_be_blocked")
    .generate()
    .unwrap()

or

$ bindgen input.h --blocklist-var should_be_blocked

Actual Results

There's no method to blocklist a variable.

Expected Results

blocklist_var is the only missing allowlist_* mirror method. I don't yet see any use besides having parity with allowlist. I have a patch ready, if we decide to implement this.

[emilio]

Fixed by #2731