samples: migrate includes to contain <zephyr/...> prefix

In order to bring consistency in-tree, migrate all samples to the use
the new prefix <zephyr/...>. Note that the conversion has been scripted:

```python
from pathlib import Path
import re

EXTENSIONS = ("c", "h", "cpp", "rst")

for p in Path(".").glob("samples/**/*"):
    if not p.is_file() or p.suffix and p.suffix[1:] not in EXTENSIONS:
        continue

    content = ""
    with open(p) as f:
        for line in f:
            m = re.match(r"^(.*)#include <(.*)>(.*)$", line)
            if (m and
                not m.group(2).startswith("zephyr/") and
                (Path(".") / "include" / "zephyr" / m.group(2)).exists()):
                content += (
                    m.group(1) +
                    "#include <zephyr/" + m.group(2) +">" +
                    m.group(3) + "\n"
                )
            else:
                content += line

    with open(p, "w") as f:
        f.write(content)
```

Signed-off-by: Gerard Marull-Paretas <gerard.marull@nordicsemi.no>