Fix a build failure with musl

```
compiling gc.c
In file included from gc.c:80:
/usr/include/sys/prctl.h:88:8: error: redefinition of 'struct prctl_mm_map'
   88 | struct prctl_mm_map {
      |        ^~~~~~~~~~~~
In file included from gc.c:79:
/usr/include/linux/prctl.h:134:8: note: originally defined here
  134 | struct prctl_mm_map {
      |        ^~~~~~~~~~~~
```

The first include is not needed and is what causes this issue.
Two other places in ruby exclusively use the sys import.

See https://github.com/seccomp/libseccomp/issues/19 for a similar problem.

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