serial: core: drop unnecessary gpio include

Drop the recently added gpio include from the serial-core header in
favour of a forward declaration and instead include the gpio header only
where needed.

Signed-off-by: Johan Hovold <johan@kernel.org>
Link: https://lore.kernel.org/r/20200610155121.14014-1-johan@kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/8250/8250_port.c

@@ -16,6 +16,7 @@
 #include <linux/ioport.h>
 #include <linux/init.h>
 #include <linux/console.h>
+#include <linux/gpio/consumer.h>
 #include <linux/sysrq.h>
 #include <linux/delay.h>
 #include <linux/platform_device.h>

drivers/tty/serial/serial_core.c

@@ -14,6 +14,7 @@
 #include <linux/sched/signal.h>
 #include <linux/init.h>
 #include <linux/console.h>
+#include <linux/gpio/consumer.h>
 #include <linux/of.h>
 #include <linux/proc_fs.h>
 #include <linux/seq_file.h>

include/linux/serial_core.h

@@ -10,7 +10,6 @@
 #include <linux/bitops.h>
 #include <linux/compiler.h>
 #include <linux/console.h>
-#include <linux/gpio/consumer.h>
 #include <linux/interrupt.h>
 #include <linux/circ_buf.h>
 #include <linux/spinlock.h>
@@ -30,6 +29,7 @@
 struct uart_port;
 struct serial_struct;
 struct device;
+struct gpio_desc;
 
 /*
  * This structure describes all the operations that can be done on the