s390/boot: disable address-of-packed-member warning

Get rid of gcc9 warnings like this:

arch/s390/boot/ipl_report.c: In function 'find_bootdata_space':
arch/s390/boot/ipl_report.c:42:26: warning: taking address of packed member of 'struct ipl_rb_components' may result in an unaligned pointer value [-Waddress-of-packed-member]
   42 |  for_each_rb_entry(comp, comps)
      |                          ^~~~~

This is effectively the s390 variant of commit 20c6c189


("x86/boot: Disable the address-of-packed-member compiler warning").

Reviewed-by: Vasily Gorbik <gor@linux.ibm.com>
Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>