libc: fix ntohl() and friends under mode=release

The glibc headers define those as macros, need to undefine before declaring
as functions.

libc/network/htonl.c

 #include <netinet/in.h>
 #include <byteswap.h>
 
+#undef htonl
 uint32_t htonl(uint32_t n)
 {
 	union { int i; char c; } u = { 1 };

libc/network/htons.c

 #include <netinet/in.h>
 #include <byteswap.h>
 
+#undef htons
 uint16_t htons(uint16_t n)
 {
 	union { int i; char c; } u = { 1 };

libc/network/ntohl.c

 #include <netinet/in.h>
 #include <byteswap.h>
 
+#undef ntohl
 uint32_t ntohl(uint32_t n)
 {
 	union { int i; char c; } u = { 1 };

libc/network/ntohs.c

 #include <netinet/in.h>
 #include <byteswap.h>
 
+#undef ntohs
 uint16_t ntohs(uint16_t n)
 {
 	union { int i; char c; } u = { 1 };