x86: hyperv: Make it build with CONFIG_HYPERV=m again

Commit 1aec1696 (x86: Hyperv: Cleanup the irq mess) removed the
ability to build the hyperv stuff as a module. Bring it back.

Reported-by: fengguang.wu@intel.com
Signed-off-by: NThomas Gleixner <tglx@linutronix.de>
Cc: K. Y. Srinivasan <kys@microsoft.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: linuxdrivers <devel@linuxdriverproject.org>
Cc: x86 <x86@kernel.org>

arch/x86/kernel/cpu/mshyperv.c

@@ -31,7 +31,7 @@
 struct ms_hyperv_info ms_hyperv;
 EXPORT_SYMBOL_GPL(ms_hyperv);
 
-#ifdef CONFIG_HYPERV
+#if IS_ENABLED(CONFIG_HYPERV)
 static irq_handler_t *vmbus_handler;
 
 void hyperv_vector_handler(struct pt_regs *regs)