CDRIVER-427 move header include guard above checks

Seems to work including the file jeremy mentioned with this test program:

```c
#include <stdio.h>
#include "mongoc.h"
#include "mongoc-bulk-operation-private.h"

int main() {
    printf("works\n");
    return 0;
}
```

```sh
gcc -I./include/libmongoc-1.0 -I./include/libbson-1.0 test.c -DMONGOC_I_AM_A_DRIVER -o test
```