mm/mmzone.c: make "migratetype_names" const char *

Those strings are immutable in fact.

Link: http://lkml.kernel.org/r/20181124090327.GA10877@avx2


Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
Reviewed-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>