vmmon: include <asm/irq_vectors.h> explicitly

After mainline commit 13c01139b171 ("x86/headers: Remove APIC headers from
<asm/smp.h>") in 5.9-rc1, APIC headers are no longer included via
<asm/smp.h> so that linux/hostif.c will use incorrect fallback definitions
of SPURIOUS_APIC_VECTOR, POSTED_INTR_VECTOR and ERROR_APIC_VECTOR even if
built against kernel where these are defined.

Include <asm/irq_vectors.h> in linux/hostif.c explicitly to avoid that.

vmmon-only/linux/hostif.c

@@ -48,6 +48,7 @@
 #include <asm/io.h>
 #include <asm/page.h>
 #include <asm/uaccess.h>
+#include <asm/irq_vectors.h>
 #include <linux/capability.h>
 #include <linux/kthread.h>
 #include <linux/wait.h>