lorawan: move `REQUIRES_FULL_LIBC` dependency

Move the `REQUIRES_FULL_LIBC` dependency from `config LORA` to
`config LORAWAN`. The commit that added the `select` (f590d4fa) mentions
that this is required by `loramac-node`, which is only used by LoRaWAN,
not the base LoRa code.

This results in small FLASH savings when compiling the samples, but can
result in larger savings in more complex applications:
```
// With REQUIRES_FULL_LIBC
west build -b 96b_wistrio zephyr/samples/drivers/lora/send/
[162/162] Linking C executable zephyr/zephyr.elf
Memory region         Used Size  Region Size  %age Used
           FLASH:       37708 B       128 KB     28.77%
            SRAM:        8832 B        32 KB     26.95%
        IDT_LIST:          0 GB         2 KB      0.00%

// Without REQUIRES_FULL_LIBC
[181/181] Linking C executable zephyr/zephyr.elf
Memory region         Used Size  Region Size  %age Used
           FLASH:       37444 B       128 KB     28.57%
            SRAM:        8832 B        32 KB     26.95%
        IDT_LIST:          0 GB         2 KB      0.00%
```

Signed-off-by: Jordan Yates <jordan.yates@data61.csiro.au>