product/juno: Condtionally include clock header file

The mod_clock.h is not required for juno rom bypass binary causing
build error while building with flag BUILD_HAS_MOD_CLOCK, hence it
is conditionally used.

Signed-off-by: Md Golam Tanvir Zim <mdgolamtanvir.zim@arm.com>
Change-Id: I2011760d9ad8bcc325c5b641661063148402e53a