Don't populate the array cpcap_battery_irqs  on the stack but instead
make it static. Makes the object code smaller by 99 bytes:

Before:
   text	   data	    bss	    dec	    hex	filename
  13673	   2448	      0	  16121	   3ef9	cpcap-battery.o

After:
   text	   data	    bss	    dec	    hex	filename
  13510	   2512	      0	  16022	   3e96	cpcap-battery.o

(gcc version 8.2.0 x86_64)
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>