ftrace: Use objtree for C version of recordmcount

The C version of recordmcount is compiled to a binary, which will
end up located in the objtree. If the kernel is built with O=path,
the srctree will not include the binary recordmcount caller.

Cc: Michal Marek <mmarek@suse.cz>
Cc: linux-kbuild@vger.kernel.org
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>