[PATCH] Add a existence check of symbols for --dump-dmesg.

Since the format of the kernel log buffer was changed in kernel 3.5,
some symbols related to the format have been needed for --dump-dmesg
option. However, the functions for --dump-dmesg use the symbols
without existence check, so makedumpfile can run into unexpected
error.
In practice, this is a definite issue since kernel 3.5 doesn't
export SIZE(log) to VMCOREINFO.

Signed-off-by: Atsushi Kumagai <kumagai-atsushi@mxc.nes.nec.co.jp>