Warn when exporting a non-extern-C function pointer

[jrose-signal]

#[repr(C)]
pub struct Payload {
  data: [u8; 128],
}

#[repr(C)]
pub struct Example {
  callback: /* extern "C" */ fn(a: Payload, b: Payload),
}

#[unsafe(no_mangle)]
pub extern "C" fn test(input: Example) {}

cbindgen-ing this code (for C) produces

#include <stdarg.h>
#include <stdbool.h>
#include <stdint.h>
#include <stdlib.h>

typedef struct Payload {
  uint8_t data[128];
} Payload;

typedef struct Example {
  void (*callback)(struct Payload a, struct Payload b);
} Example;

void test(struct Example input);

with no warnings or errors. However, the ABI for callback isn't necessarily the same in C and Rust without the extern "C". (I recently ran into this, and was confused why my structs-passed-by-value had garbage in them until I tracked down my mistake.)

Related: #24, #55, #132.