powerpc/pseries/vas: Include irqdomain.h

There are patches in flight to break the dependency between asm/irq.h
and linux/irqdomain.h, which would break compilation of vas.c because it
needs the declaration of irq_create_mapping() etc.

So add an explicit include of irqdomain.h to avoid that becoming a
problem in future.
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20210625045337.3197833-1-mpe@ellerman.id.au

arch/powerpc/platforms/pseries/vas.c

@@ -12,6 +12,7 @@
 #include <linux/delay.h>
 #include <linux/slab.h>
 #include <linux/interrupt.h>
+#include <linux/irqdomain.h>
 #include <asm/machdep.h>
 #include <asm/hvcall.h>
 #include <asm/plpar_wrappers.h>