drivers/serial/8250_acpi.c: fix a warning

This patch fixes the following warning:

<-- snip -->

...
CC drivers/serial/8250_acpi.o
drivers/serial/8250_acpi.c: In function `acpi_serial_ext_irq':
drivers/serial/8250_acpi.c:51: warning: implicit declaration of function `acpi_register_gsi'
...

<-- snip -->

Signed-off-by Adrian Bunk <[email protected]>

drivers/serial/8250_acpi.c renamed from drivers/serial/8250_acpi.c.old +1 -0

@@ -9,6 +9,7 @@
  * (at your option) any later version.
  */
 
+#include <linux/config.h>
 #include <linux/acpi.h>
 #include <linux/init.h>
 #include <linux/module.h>