Unbreak with libgit2 >=1.9.0.

Fix for libgit2->=1.8.0:
error: use of undeclared identifier 'git_error_clear'

Index: es-app/src/guis/GuiThemeDownloader.cpp
--- es-app/src/guis/GuiThemeDownloader.cpp.orig
+++ es-app/src/guis/GuiThemeDownloader.cpp
@@ -6,6 +6,9 @@
 //  Theme downloader.
 //
 
+#include <git2/deprecated.h>
+#include <git2/sys/errors.h>
+
 #include "guis/GuiThemeDownloader.h"
 
 #include "ApplicationVersion.h"
