mips: Fix KBUILD_CPPFLAGS definition

The KBUILD_CPPFLAGS variable is no longer passed to sh -c 'gcc ...',
but exported and used by the link-vmlinux.sh script. This means that the
double-quotes will not be evaluated by the shell.

Reported-by: Paul Gortmaker <paul.gortmaker@windriver.com>
Signed-off-by: Michal Marek <mmarek@suse.cz>