efi_loader: efi_esrt: Fix the build warning for 32 bit systems

Fix the build warning when building for 32 bit systems by using the
length modifier for size_t.

lib/efi_loader/efi_esrt.c: In function ‘efi_esrt_populate’:
include/efi_loader.h:126:8: warning: format ‘%ld’ expects argument of type ‘long int’, but argument 8 has type ‘size_t’ {aka ‘unsigned int’} [-Wformat=]

Signed-off-by: Sughosh Ganu <sughosh.ganu@linaro.org>
Reviewed-by: Heinrich Schuchardt <xypron.glpk@gmx.de>