src: suppress user_timeout maybe-uninitialized

This commit initializes user_timeout in uv__io_poll to avoid a
maybe-uninitialized warning:

$ cmake .. -DCMAKE_C_FLAGS="-Wmaybe-uninitialized -O3"
...
[ 14%] Building C object CMakeFiles/uv.dir/src/unix/tty.c.o
/libuv/libuv/src/unix/linux-core.c: In function ‘uv__io_poll’:
/libuv/libuv/src/unix/linux-core.c:351:10: warning:
‘user_timeout’ may be used uninitialized in this function
[-Wmaybe-uninitialized]
  351 |       if (timeout == -1)
      |          ^

PR-URL: https://github.com/libuv/libuv/pull/2976
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>

src/unix/linux-core.c

@@ -281,6 +281,7 @@ void uv__io_poll(uv_loop_t* loop, int timeout) {
     timeout = 0;
   } else {
     reset_timeout = 0;
+    user_timeout = 0;
   }
 
   /* You could argue there is a dependency between these two but