scripts/kernel-doc: fix fatal script error

Fix fatal error in scripts/kernel-doc by ignoring the "__weak" attribute:

  Error(drivers/pci/pci.c:2820): cannot understand prototype: 'char * __weak pcibios_setup(char *str) '

Signed-off-by: Randy Dunlap <rdunlap@xenotime.net>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>