Add #include <stddef.h> to a central place (arch.h) instead of #including it in several other files throughout lwip since size_t is needed in many places

See http://lwip.100.n7.nabble.com/Issue-in-arch-h-for-lwIP-2-0-0-td27948.html

src/core/inet_chksum.c

@@ -51,7 +51,6 @@
 #include "lwip/def.h"
 #include "lwip/ip_addr.h"
 
-#include <stddef.h>
 #include <string.h>
 
 #ifndef LWIP_CHKSUM

src/include/lwip/api.h

@@ -43,8 +43,7 @@
 /* Note: Netconn API is always available when sockets are enabled -
  * sockets are implemented on top of them */
 
-#include <stddef.h> /* for size_t */
-
+#include "lwip/arch.h"
 #include "lwip/netbuf.h"
 #include "lwip/sys.h"
 #include "lwip/ip_addr.h"

src/include/lwip/arch.h

@@ -47,6 +47,8 @@
 
 #include "arch/cc.h"
 
+#include <stddef.h> /* for size_t */
+
 /** Define this to 1 in arch/cc.h of your port if your compiler does not provide
  * the stdint.h header. This cannot be \#defined in lwipopts.h since
  * this is not an option of lwIP itself, but an option of the lwIP port

src/include/lwip/mem.h

@@ -45,7 +45,8 @@ extern "C" {
 
 #if MEM_LIBC_MALLOC
 
-#include <stddef.h> /* for size_t */
+#include "lwip/arch.h"
+
 typedef size_t mem_size_t;
 #define MEM_SIZE_F SZT_F
 

src/include/lwip/netdb.h

@@ -38,8 +38,7 @@
 
 #if LWIP_DNS && LWIP_SOCKET
 
-#include <stddef.h> /* for size_t */
-
+#include "lwip/arch.h"
 #include "lwip/inet.h"
 #include "lwip/sockets.h"
 

src/include/lwip/priv/api_msg.h

@@ -43,8 +43,7 @@
 /* Note: Netconn API is always available when sockets are enabled -
  * sockets are implemented on top of them */
 
-#include <stddef.h> /* for size_t */
-
+#include "lwip/arch.h"
 #include "lwip/ip_addr.h"
 #include "lwip/err.h"
 #include "lwip/sys.h"

src/include/lwip/sockets.h

@@ -43,8 +43,6 @@
 
 #if LWIP_SOCKET /* don't build if not configured for use in lwipopts.h */
 
-#include <stddef.h> /* for size_t */
-
 #include "lwip/ip_addr.h"
 #include "lwip/err.h"
 #include "lwip/inet.h"