microblaze: Remove gcc flag for non existing early_printk.c file

early_printk support for removed long time ago but compilation flag for
ftrace still points to already removed file that's why remove that line
too.

Fixes: 96f0e6fc

 ("microblaze: remove redundant early_printk support")
Signed-off-by: Michal Simek <michal.simek@amd.com>
Link: https://lore.kernel.org/r/5493467419cd2510a32854e2807bcd263de981a0.1712823702.git.michal.simek@amd.com