ARM: omap1: fix building gpio15xx

In some randconfig builds, the asm/irq.h header is not included
in gpio15xx.c, so add an explicit include to avoid a build fialure:

In file included from arch/arm/mach-omap1/gpio15xx.c:15:
arch/arm/mach-omap1/irqs.h:99:34: error: 'NR_IRQS_LEGACY' undeclared here (not in a function)
   99 | #define IH2_BASE                (NR_IRQS_LEGACY + 32)
      |                                  ^~~~~~~~~~~~~~
arch/arm/mach-omap1/irqs.h:105:38: note: in expansion of macro 'IH2_BASE'
  105 | #define INT_MPUIO               (5 + IH2_BASE)
      |                                      ^~~~~~~~
arch/arm/mach-omap1/gpio15xx.c:28:27: note: in expansion of macro 'INT_MPUIO'
   28 |                 .start  = INT_MPUIO,
      |                           ^~~~~~~~~

Acked-by: Aaro Koskinen <aaro.koskinen@iki.fi>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>