cpan.bbclass: Fix config error while patches to Makefile.PL

cpan.bbclass: Fix config error while patches to Makefile.PL

If there is a patch to Makefile.PL, a Makefile.PL but no Makefile
will be placed in ${B}/.pc/xxx.patch/ after do_patch.
And no Makefile will be generated for *this* Makefile.PL.
While do_configure, the original code tries to sed Makefiles
matching with each Makefile.PL in {B}, so this would fail.

(From OE-Core rev: 7c99105d70a16aa9e42429224abed28743e627b2)

Signed-off-by: Xin Ouyang <Xin.Ouyang@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>