compat: use STDIN_FILENO over 0

This is already used throughout this file, and is self-documenting.

Author: fanquake

src/compat/stdin.cpp

@@ -58,7 +58,7 @@ bool StdinReady()
     return false;
 #else
     struct pollfd fds;
-    fds.fd = 0; /* this is STDIN */
+    fds.fd = STDIN_FILENO;
     fds.events = POLLIN;
     return poll(&fds, 1, 0) == 1;
 #endif