memblock test: fix implicit declaration of function 'isspace'

Commit 1e4c64b7 ("mm/memblock: Add "reserve_mem" to reserved named
memory at boot up") introduce usage of isspace().

Let's include <linux/ctype.h> in kernel.h to fix this.

Signed-off-by: Wei Yang <richard.weiyang@gmail.com>
Link: https://lore.kernel.org/r/20240806010319.29194-4-richard.weiyang@gmail.com


Signed-off-by: Mike Rapoport (Microsoft) <rppt@kernel.org>