opkg_manager: avoid the zombie apocalypse, use waitpid()

Origin commit data
------------------
Commit: 1454d09a75
Author: Stefan Seyfried <seife@tuxbox-git.slipkontur.de>
Date: 2017-01-29 (Sun, 29 Jan 2017)

src/gui/opkg_manager.cpp

@@ -781,6 +781,8 @@ void COPKGManager::getPkgData(const int pkg_content_id)
 		}
 	}
 
+	waitpid(pid, NULL, 0); /* beware of the zombie apocalypse! */
+
 	fclose(f);
 }