linux/coredump.h needs asm/siginfo.h

Commit 5ab1c309 ("coredump: pass siginfo_t* to do_coredump() and
below, not merely signr") added siginfo_t to linux/coredump.h but forgot
to include asm/siginfo.h.  This breaks the build for UML/i386.  (And any
other arch where asm/siginfo.h is not magically preincluded...)

  In file included from arch/x86/um/elfcore.c:2:0: include/linux/coredump.h:15:25: error: unknown type name 'siginfo_t'
  make[1]: *** [arch/x86/um/elfcore.o] Error 1

Signed-off-by: Richard Weinberger <richard@nod.at>
Cc: Denys Vlasenko <vda.linux@googlemail.com>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Amerigo Wang <amwang@redhat.com>
Cc: "Jonathan M. Foote" <jmfoote@cert.org>
Cc: Roland McGrath <roland@hack.frob.com>
Cc: Pedro Alves <palves@redhat.com>
Cc: Fengguang Wu <fengguang.wu@intel.com>
Cc: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>