objtool: Initialize variable to silence old compiler

gcc version 4.1.2 20080704 (Red Hat 4.1.2-55) barfs with:

    CC       /tmp/build/objtool/builtin-check.o
  cc1: warnings being treated as errors
  builtin-check.c: In function 'cmd_check':
  builtin-check.c:667: warning: 'prev_rela' may be used uninitialized in this function
  mv: cannot stat `/tmp/build/objtool/.builtin-check.o.tmp': No such file or directory
  make[1]: *** [/tmp/build/objtool/builtin-check.o] Error 1

Init it to NULL to silence it.

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Wang Nan <wangnan0@huawei.com>
Link: http://lkml.kernel.org/n/tip-qolo31rl2ojlwj1lj9dhemyz@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>