efi_loader: wrong printf format in efi_image_parse

Commit 1b6c0854 ("efi_loader: image_loader: replace debug to
EFI_PRINT") leads to a build warning on 32bit systems:

lib/efi_loader/efi_image_loader.c: In function ‘efi_image_parse’:
include/efi_loader.h:123:8: warning: format ‘%lu’ expects argument
of type ‘long unsigned int’, but argument 8 has
type ‘size_t’ {aka ‘unsigned int’} [-Wformat=]

Use %zu for printing size_t.

Fixes: 1b6c0854 ("efi_loader: image_loader: replace debug to
EFI_PRINT")

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>