[rygel] core: Fix compiler warning

[Jens Georg <jensgeorg src gnome org>]

commit 5351e79d6d69f19aab123522f8e93230f70ea505
Author: Jens Georg <mail jensge org>
Date:   Sun Apr 30 11:44:41 2017 +0200

    core: Fix compiler warning

 src/librygel-core/rygel-energy-management-helper.c |    1 +
 1 files changed, 1 insertions(+), 0 deletions(-)
---
diff --git a/src/librygel-core/rygel-energy-management-helper.c 
b/src/librygel-core/rygel-energy-management-helper.c
index c5054f8..8883173 100644
--- a/src/librygel-core/rygel-energy-management-helper.c
+++ b/src/librygel-core/rygel-energy-management-helper.c
@@ -32,6 +32,7 @@
 #include <sys/socket.h>
 #include <stdlib.h>
 #include <errno.h>
+#include <unistd.h>
 
 #include <linux/wireless.h>