all: add missing limits.h includes

Removal of kernel_types.h make these necessary.

sys/net/gnrc/netif/gnrc_netif.c

@@ -16,6 +16,7 @@
  */
 
 #include <assert.h>
+#include <limits.h>
 #include <string.h>
 #include <kernel_defines.h>
 

sys/shell/commands/sc_gnrc_icmpv6_echo.c

@@ -20,6 +20,7 @@
  */
 
 #ifdef MODULE_GNRC_ICMPV6
+#include <limits.h>
 #include <stdio.h>
 #include <stdint.h>
 #include <stdlib.h>

sys/shell/commands/sc_gnrc_netif.c

@@ -20,6 +20,7 @@
 
 #include <assert.h>
 #include <ctype.h>
+#include <limits.h>
 #include <stdio.h>
 #include <string.h>