vmw_balloon: Explicitly include linux/io.h for virt_to_phys()

Through a labyrinthian sequence of includes, usage of virt_to_phys() is
dependent on the include of asm/io.h in x86's asm/realmode.h, which is
included in x86's asm/acpi.h and thus by linux/acpi.h.  Explicitly
include linux/io.h to break the dependency on realmode.h so that a
future patch can remove the realmode.h include from acpi.h without
breaking the build.
Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
Link: https://lkml.kernel.org/r/20191126165417.22423-9-sean.j.christopherson@intel.comSigned-off-by: Ingo Molnar <mingo@kernel.org>

drivers/misc/vmw_balloon.c

@@ -17,6 +17,7 @@
 #define pr_fmt(fmt) KBUILD_MODNAME ": " fmt
 
 #include <linux/types.h>
+#include <linux/io.h>
 #include <linux/kernel.h>
 #include <linux/mm.h>
 #include <linux/vmalloc.h>