Fix build failure for gp2ap020a00f.c

drivers/built-in.o: In function `gp2ap020a00f_thresh_event_handler':
powercap_sys.c:(.text+0x15f90c): undefined reference to `irq_work_queue'
make[1]: *** [vmlinux] Error 1
make[1]: Target `uImage' not remade because of errors.
make: *** [sub-make] Error 2
make: Target `uImage' not remade because of errors.

You need the IRQ work support, but GP2AP020A00F is not selecting this
symbol.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>