sd-boot: fix -Wpointer-sign warning

src/boot/efi/boot.c

@@ -1893,8 +1893,8 @@ static VOID config_entry_add_linux(
                 UINTN bufsize = sizeof buf;
                 EFI_FILE_INFO *f;
                 CHAR8 *sections[] = {
-                        (UINT8 *)".osrel",
-                        (UINT8 *)".cmdline",
+                        (CHAR8 *)".osrel",
+                        (CHAR8 *)".cmdline",
                         NULL
                 };
                 UINTN offs[ELEMENTSOF(sections)-1] = {};

src/boot/efi/stub.c

@@ -22,10 +22,10 @@ EFI_STATUS efi_main(EFI_HANDLE image, EFI_SYSTEM_TABLE *sys_table) {
         UINTN size;
         BOOLEAN secure = FALSE;
         CHAR8 *sections[] = {
-                (UINT8 *)".cmdline",
-                (UINT8 *)".linux",
-                (UINT8 *)".initrd",
-                (UINT8 *)".splash",
+                (CHAR8 *)".cmdline",
+                (CHAR8 *)".linux",
+                (CHAR8 *)".initrd",
+                (CHAR8 *)".splash",
                 NULL
         };
         UINTN addrs[ELEMENTSOF(sections)-1] = {};