drm_hwcomposer: Fix missing header

Signed-off-by: David Riley <davidriley@google.com>
diff --git a/hwcomposer_import_nv_gralloc.cpp b/hwcomposer_import_nv_gralloc.cpp
index f012aab..c9c265d 100644
--- a/hwcomposer_import_nv_gralloc.cpp
+++ b/hwcomposer_import_nv_gralloc.cpp
@@ -19,6 +19,7 @@
 #include <cutils/log.h>
 #include <hardware/gralloc.h>
 
+#include <stdlib.h>
 #include <xf86drm.h>
 #include <xf86drmMode.h>
 #include "drm_hwcomposer.h"