linux: fix CIP RT kernel on microblaze

Fix the following build failure with CIP RT kernel on microblaze raised
since bump to version 5.10.83-cip1-rt1 in commit
b5778d7b and
https://git.kernel.org/pub/scm/linux/kernel/git/cip/linux-cip.git/commit/?id=32ac92b5a39e2ef91308929bf5ed0804094b4183:

arch/microblaze/mm/init.c:71:2: error: #endif without #if
   71 | #endif /* CONFIG_HIGHMEM */
      |  ^~~~~

Fixes:
 - http://autobuild.buildroot.org/results/27291870cf7539d26e45c45f34322d24a6dbca33



Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>