powerpc/syscall: Force inlining of __prep_irq_for_enabled_exit()

As reported by kernel test robot, a randconfig with high amount of
debuging options can lead to build failure for undefined reference
to replay_soft_interrupts() on ppc32.

This is due to gcc not seeing that __prep_irq_for_enabled_exit()
always returns true on ppc32 because it doesn't inline it for
some reason.

Force inlining of __prep_irq_for_enabled_exit() to fix the build.

Fixes: 344bb20b ("powerpc/syscall: Make interrupt.c buildable on PPC32")
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Acked-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/53f3a1f719441761000c41154602bf097d4350b5.1614148356.git.christophe.leroy@csgroup.eu