memblock test: fix implicit declaration of function 'isspace'

Commit 1e4c64b71c9b ("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 <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Mike Rapoport (Microsoft) <[e-mail]>

tools/testing/memblock/linux/kernel.h

···
 #include <linux/linkage.h>
 #include <linux/kconfig.h>
 #include <linux/string.h>
+#include <linux/ctype.h>
 
 #endif