power: sequencing: thead-gpu: add missing header

When using kcalloc(), kfree() etc., we need to include linux/slab.h.
While on some architectures it may work fine because the header is
pulled in implicitly, on others it triggers the following errors:

drivers/power/sequencing/pwrseq-thead-gpu.c: In function ‘pwrseq_thead_gpu_match’:
drivers/power/sequencing/pwrseq-thead-gpu.c:147:21: error: implicit declaration of function ‘kcalloc’ [-Wimplicit-function-declaration]
  147 |         ctx->clks = kcalloc(ctx->num_clks, sizeof(*ctx->clks), GFP_KERNEL);

Fixes: d4c2d9b5 ("power: sequencing: Add T-HEAD TH1520 GPU power sequencer driver")
Reviewed-by: Ulf Hansson <ulf.hansson@linaro.org>
Link: https://lore.kernel.org/r/20250624-pwrseq-match-defines-v1-1-a59d90a951f1@linaro.org


Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>