libgui: Fix missing va_end in Surface::hook_perform()

BUG=23667255
TEST=libgui builds and volantis still works

Change-Id: I01de6ade6a47072c3a32c3eedc8568b1fd571363
diff --git a/libs/gui/Surface.cpp b/libs/gui/Surface.cpp
index 27f7ed6..de5c275 100644
--- a/libs/gui/Surface.cpp
+++ b/libs/gui/Surface.cpp
@@ -184,7 +184,9 @@
     va_list args;
     va_start(args, operation);
     Surface* c = getSelf(window);
-    return c->perform(operation, args);
+    int result = c->perform(operation, args);
+    va_end(args);
+    return result;
 }
 
 int Surface::setSwapInterval(int interval) {