clk: rockchip: rk3576: add missing slab.h include

clk: rockchip: rk3576: add missing slab.h include

The change for auxiliary GRFs introduced kzalloc usage into the rk3576 clock
driver, but missed adding the header for its prototype. Add it now.

Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202505150941.KWKskr2c-lkp@intel.com/
Fixes: 70a114daf207 ("clk: rockchip: introduce auxiliary GRFs")
Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Link: https://lore.kernel.org/r/20250515082652.2503063-1-heiko@sntech.de