x86: move the NUM_ISA_IRQS symbol from atpic.c into x86/isa/icu.h

This is not the best location, but works for now.

Reviewed by:	markj
Sponsored by:	The FreeBSD Foundation
MFC after:	1 week
Differential revision:	https://reviews.freebsd.org/D56003

sys/x86/isa/atpic.c

@@ -68,8 +68,6 @@
 
 #define	IMEN_MASK(ai)		(IRQ_MASK((ai)->at_irq))
 
-#define	NUM_ISA_IRQS		16
-
 static void	atpic_init(void *dummy);
 
 inthand_t

sys/x86/isa/icu.h

@@ -71,4 +71,6 @@
 void	atpic_handle_intr(u_int vector, struct trapframe *frame);
 void	atpic_startup(void);
 
+#define	NUM_ISA_IRQS		16
+
 #endif /* !_X86_ISA_ICU_H_ */