zlib: Fix build error when LOG_LEVEL=50
zlib: Fix build error when LOG_LEVEL=50

When enabling VERBOSE() traces, the zlib library fails to compile
because of an incompatible format specifier string. Fix that.

Change-Id: I74ff1c8dc2e6157ee982f7754bce4504599e3013
Signed-off-by: Sandrine Bailleux <4b69fcfca8da45b0c11ecda5434c5da526575d9d@arm.com>

achingupta/arm-trusted-firmware,achingupta/arm-trusted-firmware