esl-iter.h: fix type declaration of return value for gcc-13

[trofi]

Upcoming gcc-13 detected return type mismatch as:

esl-iter.c:84:1: error: conflicting types for 'esl_iter_next_with_size_correction' due to enum/integer mismatch; have 'esl_iter_status_t(esl_iter *, efi_guid_t *, efi_guid_t *, uint8_t **, size_t *, _Bool)' {aka 'enum esl_iter_status(esl_iter *, efi_guid_t *, efi_guid_t *, unsigned char **, long unsigned int *, _Bool)'} [-Werror=enum-int-mismatch]
   84 | esl_iter_next_with_size_correction(esl_iter *iter, efi_guid_t *type,
      | ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
In file included from efisec.h:25,
                 from esl-iter.c:7:
esl-iter.h:61:12: note: previous declaration of 'esl_iter_next_with_size_correction' with type 'int(esl_iter *, efi
*, efi_guid_t *, uint8_t **, size_t *, _Bool)' {aka 'int(esl_iter *, efi_guid_t *, efi_guid_t *, unsigned char **, long unsigned int *, _Bool)'}
   61 | extern int esl_iter_next_with_size_correction(esl_iter *iter, efi_guid_t *type,
      |            ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

The change brings declaration and definition in sync.