aix: fix -Wmaybe-uninitialized warning

PR-URL: https://github.com/libuv/libuv/pull/1650
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>

src/unix/aix.c

@@ -921,6 +921,7 @@ int uv_uptime(double* uptime) {
   size_t entries = 0;
   time_t boot_time;
 
+  boot_time = 0;
   utmpname(UTMP_FILE);
 
   setutent();