executor: move syz_execute_func after os imports. (#1107)

This commit moves the definition of the `syz_execute_func` after the
block of code that imports all the OS specific common headers.

This is required because after commit
dfd3394d42ddd333c68cf355273b312da8c65a51 `syz_execute_func` started
using the `NONFAILING` macro, which is defined in those header files for
each OS.

I also ran `make generate`.

TEST=I only tested that the executor works for Fuchsia with:

```shell
$ make executor TARGETOS=fuchsia TARGETARCH=amd64 SOURCEDIR=~/fuchsia
```

0 files changed, 0 insertions, 0 deletions