clock-gcc-8974: Change debug_mux_priv to static

debug_mux_priv should not be exposed outside of clock-gcc-8974.c. Add
static to fix multiple definition errors.

Change-Id: Ica5d978a27b42c1b3d4260957f6e388196f5cd90
Signed-off-by: Junjie Wu <junjiew@codeaurora.org>