sd-boot: efi-log: fix `__stack_chk_guard` type

sd-boot: efi-log: fix `__stack_chk_guard` type

In https://gcc.gnu.org/PR121911 `gcc` started enforcing the type of
`__stack_chk_guard` to `uintptr_t` and broke `systemd` build as:

```
../src/boot/efi-log.c:136:17: error: conflicting types for '__stack_chk_guard'; have 'intptr_t' {aka 'long int'}
  136 | _used_ intptr_t __stack_chk_guard = (intptr_t) 0x70f6967de78acae3;
      |                 ^~~~~~~~~~~~~~~~~
cc1: note: previous declaration of '__stack_chk_guard' with type 'long unsigned int'
../src/boot/efi-log.c:136:17: error: declaration of '__stack_chk_guard' shadows a global declaration [-Werror=shadow]
  136 | _used_ intptr_t __stack_chk_guard = (intptr_t) 0x70f6967de78acae3;
      |                 ^~~~~~~~~~~~~~~~~
```

Let's match the declaration to unsigned type as suggested by upstream in
https://gcc.gnu.org/PR121911#c6.