Fix missing errno.h header

Fixes this compile error:

```
../src/cc/frontends/clang/kbuild_helper.h:40:64: error: use of undeclared identifier 'errno'
      fprintf(stderr, "chdir(%s): %s\n", dst.c_str(), strerror(errno));
```

src/cc/frontends/clang/kbuild_helper.h

@@ -18,6 +18,7 @@
 #include <string>
 #include <vector>
 #include <unistd.h>
+#include <errno.h>
 
 namespace ebpf {