clk: at91: Include clk.h and slab.h

This clock provider uses the consumer API, so include clk.h
explicitly. Also include slab.h instead of relying on clkdev.h
for it.
Acked-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>

drivers/clk/at91/clk-slow.c

@@ -10,8 +10,10 @@
  *
  */
 
+#include <linux/clk.h>
 #include <linux/clk-provider.h>
 #include <linux/clkdev.h>
+#include <linux/slab.h>
 #include <linux/clk/at91_pmc.h>
 #include <linux/delay.h>
 #include <linux/of.h>