mkeficapsule: Correct printf() strings

Use %z when printing size_t values. This avoids errors on 32-bit
machines.

Signed-off-by: Simon Glass <sjg@chromium.org>
Use a conversion to size_t for printing stat.st_size.
Reviewed-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>