[gegl] Fix build with OpenEXR 3

[Øyvind "pippin" Kolås <ok src gnome org>]

commit 499a239d158fadb3a04499255b5b282a8a6023bb
Author: Antonio Rojas <arojas archlinux org>
Date:   Sat Apr 24 10:51:09 2021 +0000

    Fix build with OpenEXR 3
    
    Add a header that is no longer transitively included

 operations/external/exr-save.cc | 1 +
 1 file changed, 1 insertion(+)
---
diff --git a/operations/external/exr-save.cc b/operations/external/exr-save.cc
index 1e8c09d96..87abed511 100644
--- a/operations/external/exr-save.cc
+++ b/operations/external/exr-save.cc
@@ -45,6 +45,7 @@ extern "C" {
 #include <ImfChromaticities.h>
 #include <ImfStandardAttributes.h>
 #include <ImfArray.h>
+#include <ImfFrameBuffer.h>
 #include "ImathRandom.h"