coresight: syscfg: Fix compiler warning

This fixes warnings with -Wimplicit-function-declaration, e.g.

drivers/hwtracing/coresight/coresight-syscfg.c:455:15: error:
implicit declaration of function 'kzalloc' [-Werror,
-Wimplicit-function-declaration]
        csdev_item = kzalloc(sizeof(struct cscfg_registered_csdev),
                             GFP_KERNEL);

Link: https://lore.kernel.org/r/20210830172820.2840433-1-jiancai@google.com
Fixes: 85e2414c51 ("coresight: syscfg: Initial coresight system configuration")
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Jian Cai <jiancai@google.com>
Signed-off-by: Mathieu Poirier <mathieu.poirier@linaro.org>
Link: https://lore.kernel.org/r/20210913164613.1675791-2-mathieu.poirier@linaro.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/hwtracing/coresight/coresight-syscfg.c

@@ -5,6 +5,7 @@
  */
 
 #include <linux/platform_device.h>
+#include <linux/slab.h>
 
 #include "coresight-config.h"
 #include "coresight-etm-perf.h"