I think this is fixed
byhttps://github.com/seebk/GIMP-Lensfun/commit/ca4511c1a4dd8edabe86e4a943861fda07b7e86c
Feel free to 0day NMU, I don't have much time right now.
I did a test build with that commit added as a quilt patch and it built
successfully, so I went ahead with the 0-day NMU. Debdiff is attatched.
diff -Nru gimplensfun-0.2.4/debian/changelog gimplensfun-0.2.4/debian/changelog
--- gimplensfun-0.2.4/debian/changelog 2016-04-16 17:49:36.000000000 +0000
+++ gimplensfun-0.2.4/debian/changelog 2020-02-05 23:52:20.000000000 +0000
@@ -1,3 +1,11 @@
+gimplensfun (0.2.4-1.1) unstable; urgency=medium
+
+ * Non-maintainer upload.
+ * Add upstream patch to add a missing #include and hence
+ fix build with new exiv2 (Closes: 950172).
+
+ -- Peter Michael Green <plugw...@debian.org> Wed, 05 Feb 2020 23:52:20 +0000
+
gimplensfun (0.2.4-1) unstable; urgency=medium
* New upstream release
diff -Nru gimplensfun-0.2.4/debian/patches/02-add-missing-header.patch
gimplensfun-0.2.4/debian/patches/02-add-missing-header.patch
--- gimplensfun-0.2.4/debian/patches/02-add-missing-header.patch
1970-01-01 00:00:00.000000000 +0000
+++ gimplensfun-0.2.4/debian/patches/02-add-missing-header.patch
2020-02-05 23:51:42.000000000 +0000
@@ -0,0 +1,18 @@
+commit ca4511c1a4dd8edabe86e4a943861fda07b7e86c
+Author: Neil Mayhew <n...@neil.mayhew.name>
+Date: Fri Jul 5 13:37:39 2019 -0600
+
+ Add missing header file
+
+diff --git a/src/gimplensfun.cpp b/src/gimplensfun.cpp
+index da2a94c..8e9e12e 100644
+--- a/src/gimplensfun.cpp
++++ b/src/gimplensfun.cpp
+@@ -31,6 +31,7 @@
+ #include <libgimp/gimp.h>
+ #include <libgimp/gimpui.h>
+
++#include <exiv2/error.hpp>
+ #include <exiv2/image.hpp>
+ #include <exiv2/exif.hpp>
+
diff -Nru gimplensfun-0.2.4/debian/patches/series
gimplensfun-0.2.4/debian/patches/series
--- gimplensfun-0.2.4/debian/patches/series 2015-12-26 19:45:21.000000000
+0000
+++ gimplensfun-0.2.4/debian/patches/series 2020-02-05 23:51:59.000000000
+0000
@@ -1 +1,2 @@
01-debian-cflags-ldflags.patch
+02-add-missing-header.patch
