pinctrl: stmfx: 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/pinctrl/pinctrl-stmfx.c

@@ -10,6 +10,8 @@
 #include <linux/mfd/stmfx.h>
 #include <linux/module.h>
 #include <linux/platform_device.h>
+#include <linux/seq_file.h>
+
 #include <linux/pinctrl/pinconf.h>
 #include <linux/pinctrl/pinmux.h>