clk: ccf: Add missing #include <dm/uclass.h> to clk-mux.c

After adding custom get_rate helper function it was necessary to include
<dm/uclass.h> to avoid warnings about missing uclass_get_device_by_name.

Signed-off-by: Lukasz Majewski <lukma@denx.de>
Series-to: u-boot

drivers/clk/clk-mux.c

@@ -26,6 +26,7 @@
 #include <clk-uclass.h>
 #include <dm/device.h>
 #include <dm/devres.h>
+#include <dm/uclass.h>
 #include <linux/bitops.h>
 #include <malloc.h>
 #include <asm/io.h>