unix: fix uv_uptime() regression

As it was in the original version of
https://github.com/libuv/libuv/pull/3072, `%lf` must be used in
`sscanf()` as the value is being stored in a `double` and not a `float`.

PR-URL: https://github.com/libuv/libuv/pull/3080
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>

src/unix/linux-core.c

@@ -636,7 +636,7 @@ int uv_uptime(double* uptime) {
   /* Try /proc/uptime first, then fallback to clock_gettime(). */
   
   if (0 == uv__slurp("/proc/uptime", buf, sizeof(buf)))
-    if (1 == sscanf(buf, "%f", uptime))
+    if (1 == sscanf(buf, "%lf", uptime))
       return 0;
 
   /* Try CLOCK_BOOTTIME first, fall back to CLOCK_MONOTONIC if not available