clk: test: Forward-declare struct of_phandle_args in kunit/clk.h

clk: test: Forward-declare struct of_phandle_args in kunit/clk.h

Add a forward-declare of struct of_phandle_args to prevent the compiler
warning:

../include/kunit/clk.h:29:63: warning: ‘struct of_phandle_args’ declared
inside parameter list will not be visible outside of this definition or
declaration
   struct clk_hw *(*get)(struct of_phandle_args *clkspec, void *data),

Signed-off-by: Richard Fitzgerald <rf@opensource.cirrus.com>
Link: https://lore.kernel.org/r/20250327125214.82598-1-rf@opensource.cirrus.com
Fixes: a82fcb16d977 ("clk: test: Add test managed of_clk_add_hw_provider()")
Signed-off-by: Stephen Boyd <sboyd@kernel.org>