hyperv: fix warning about missing prototype

Compiling with warnings enabled finds missing prototype for
hv_do_hypercall.
Signed-off-by: Stephen Hemminger <sthemmin@microsoft.com>
Signed-off-by: K. Y. Srinivasan <kys@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

arch/x86/hyperv/hv_init.c

@@ -25,7 +25,7 @@
 #include <linux/vmalloc.h>
 #include <linux/mm.h>
 #include <linux/clockchips.h>
-
+#include <linux/hyperv.h>
 
 #ifdef CONFIG_X86_64