hexagon: irq: add prototype for arch_do_IRQ()

Clang warns:

  arch/hexagon/kernel/vm_events.c:83:6: warning: no previous prototype for function 'arch_do_IRQ' [-Wmissing-prototypes]
     83 | void arch_do_IRQ(struct pt_regs *regs)
        |      ^
  arch/hexagon/kernel/vm_events.c:83:1: note: declare 'static' if the function is not intended to be used outside of this translation unit
     83 | void arch_do_IRQ(struct pt_regs *regs)
        | ^
        | static

This function is only called from assembly but the irq header is a
reasonable place to put a prototype to silence the warning.

Link: https://lkml.kernel.org/r/20231130-hexagon-missing-prototypes-v1-17-5c34714afe9e@kernel.orgSigned-off-by: Nathan Chancellor <nathan@kernel.org>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Brian Cain <bcain@quicinc.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

arch/hexagon/include/asm/irq.h

@@ -20,4 +20,7 @@
 
 #include <asm-generic/irq.h>
 
+struct pt_regs;
+void arch_do_IRQ(struct pt_regs *);
+
 #endif