Require sys/socket.h (for Cosmopolitan)

Compilation of mirb with Cosmopolitan fails because of missing include
(Cosmopolitan seems to be more strict than "traditional" compilers.

mrbgems/mruby-bin-mirb/tools/mirb/mirb_term.c

@@ -13,6 +13,7 @@
 #include <unistd.h>
 #include <termios.h>
 #include <sys/ioctl.h>
+#include <sys/select.h>
 #include <errno.h>
 
 /*