[SERIAL] Fix serial includes for modversions/modules.

This fixes the build error that occurs if you have a certain selection
of module/modversions settings.

drivers/serial/8250.c

@@ -32,6 +32,7 @@
 #include <linux/sysrq.h>
 #include <linux/serial_reg.h>
 #include <linux/serialP.h>
+#include <linux/serial.h>
 #include <linux/delay.h>
 
 #include <asm/io.h>