Include malloc.h instead of stdlib.h for mallinfo()

This change also removes the malloc.h inclusion in stdlib.h
to break the build if there are still users of mallinfo() with stdlib.h.

include/nuttx/kmalloc.h

@@ -28,6 +28,7 @@
 #include <nuttx/config.h>
 
 #include <sys/types.h>
+#include <malloc.h>
 #include <stdbool.h>
 #include <stdlib.h>
 

include/stdlib.h

@@ -30,7 +30,6 @@
 
 #include <sys/types.h>
 #include <errno.h>
-#include <malloc.h>
 #include <stdint.h>
 #include <limits.h>
 

mm/mm_heap/mm_mallinfo.c

@@ -39,7 +39,7 @@
 
 #include <nuttx/config.h>
 
-#include <stdlib.h>
+#include <malloc.h>
 #include <assert.h>
 #include <debug.h>
 

mm/umm_heap/umm_mallinfo.c

@@ -24,7 +24,7 @@
 
 #include <nuttx/config.h>
 
-#include <stdlib.h>
+#include <malloc.h>
 
 #include <nuttx/mm/mm.h>