mfd: Fix stmpe build warning

This fixes:

drivers/mfd/stmpe.c:114:1: warning: data definition has no type or storage
class [enabled by default]
drivers/mfd/stmpe.c:114:1: warning: type defaults to 'int' in declaration of 'EXPORT_SYMBOL_GPL' [-Wimplicit-int]

Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>

drivers/mfd/stmpe.c

@@ -8,6 +8,7 @@
  */
 
 #include <linux/gpio.h>
+#include <linux/export.h>
 #include <linux/kernel.h>
 #include <linux/interrupt.h>
 #include <linux/irq.h>