Fix some "little" build problems, and a redundancy call to "lwip_stats.link.recv++;" in low_level_input() & ethernetif_input().

If LINK_STATS was defined, tcpip.c couldn't be build.
Even if IP_FRAG or IP_REASSEMBLY were set, ip_frag.c functions are not build.

src/core/ipv4/ip_frag.c

@@ -37,8 +37,6 @@
  * 
  */
 
-#if (IP_FRAG || IP_REASSEMBLY)
-
 #include <string.h>
 
 #include "lwip/opt.h"
@@ -48,6 +46,8 @@
 #include "lwip/snmp.h"
 #include "lwip/stats.h"
 
+#if (IP_FRAG || IP_REASSEMBLY)
+
 static u8_t ip_reassbuf[IP_HLEN + IP_REASS_BUFSIZE];
 static u8_t ip_reassbitmap[IP_REASS_BUFSIZE / (8 * 8) + 1];
 static const u8_t bitmap_bits[8] = { 0xff, 0x7f, 0x3f, 0x1f,