portability: add sys/socket.h which was accidentally forgotten

fixes ecfdd059 "portability, libfetch: add SOCK_NONBLOCK and SOCK_CLOEXEC emulation"

portability/sys/socket.h

+#include_next <sys/socket.h>
+
+#ifdef NEED_SOCK_CLOEXEC
+#define SOCK_CLOEXEC   02000000
+#define SOCK_NONBLOCK  04000
+
+int __portable_socket(int domain, int type, int protocol);
+#define socket(...) __portable_socket(__VA_ARGS__)
+#endif