x86/iopl: Include prototype header for ksys_ioperm()

.. in order to fix a -Wmissing-prototype warning.

No functional change.
Signed-off-by: Benjamin Thiel <b.thiel@posteo.de>
Signed-off-by: Borislav Petkov <bp@suse.de>
Link: https://lkml.kernel.org/r/20200123133051.5974-1-b.thiel@posteo.de

arch/x86/kernel/ioport.c

@@ -13,6 +13,7 @@
 
 #include <asm/io_bitmap.h>
 #include <asm/desc.h>
+#include <asm/syscalls.h>
 
 #ifdef CONFIG_X86_IOPL_IOPERM