MIPS: Fix distclean with Makefile.postlink

MIPS: Fix distclean with Makefile.postlink

The postlink Makefile must include include/config/auto.conf to get the
kernel configuration variables. But in a clean kernel directory this
file does not exist, causing make to bail with the error:

arch/mips/Makefile.postlink:10: include/config/auto.conf: No such file or directory
make[1]: *** No rule to make target 'include/config/auto.conf'.  Stop.
Makefile:1290: recipe for target 'vmlinuxclean' failed

Fix this by using "-include" to not cause a Make error when the file
does not exist.

Fixes: 44079d3509ae ("MIPS: Use Makefile.postlink to insert relocations into vmlinux")
Signed-off-by: Matt Redfearn <matt.redfearn@imgtec.com>
Cc: Ralf Baechle <ralf@linux-mips.org>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/15136/
Signed-off-by: James Hogan <james.hogan@imgtec.com>