gpio: mockup: Add missing header(s)

Do not imply that some of the generic headers may be always included.
Instead, include explicitly what we are direct user of.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>

drivers/gpio/gpio-mockup.c

@@ -19,6 +19,7 @@
 #include <linux/module.h>
 #include <linux/platform_device.h>
 #include <linux/property.h>
+#include <linux/seq_file.h>
 #include <linux/slab.h>
 #include <linux/string_helpers.h>
 #include <linux/uaccess.h>