hw/arm_gic.c: Define .class_size in arm_gic_info TypeInfo

Add the missing .class_size definition to the arm_gic_info TypeInfo.
This fixes the memory corruption and possible segfault that otherwise
results when the class struct is allocated at too small a size and
the class init function writes off the end of it.

Reported-by: Adam Lackorzynski <adam@os.inf.tu-dresden.de>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
