memblock test: fix implicit declaration of function 'isspace'

Commit 1e4c64b ("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>

Author: RichardWeiYang

tools/testing/memblock/linux/kernel.h

@@ -9,5 +9,6 @@
 #include <linux/linkage.h>
 #include <linux/kconfig.h>
 #include <linux/string.h>
+#include <linux/ctype.h>
 
 #endif