libpng: gcc7 ready
libpng: gcc7 ready

Updates compiler flags to make libpng gcc7 ready.

Reviewed-by: Igor Opaniuk <ca782e5848fbe7ab0a250faa8ae33b643dd89a09@linaro.org>
Signed-off-by: Jens Wiklander <7706914404370d7502c27a0eff493dcf491feb51@linaro.org>

pascal-brand-st-dev/optee_os,pascal-brand-st-dev/optee_os,pascal-brand-st-dev/optee_os,pascal-brand-st-dev/optee_os,pascal-brand-st-dev/optee_os