DREDD injects header before Feature Test Macro

Minimal Reproducible Example (Produced with CReduce):
```
#define _GNU_SOURCE
#include <sys/mman.h>
void main() {
  size_t a = 4096;
  mremap;
}
```

## Environment
- OS: Ubuntu 20.04.6 LTS
- Compiler: gcc (Ubuntu 9.4.0-1ubuntu1~20.04.2) 9.4.0
- C library: (Ubuntu GLIBC 2.31-0ubuntu9.14) 2.31