snapcraft.yaml: stage lensfun

liblensfun-dev is a `build-package`, but without actually staging the
lensfun database Darktable is unable to actually do its lens correction
at runtime.

Add liblensfun0 as a `stage-package`.

Fix #5

Signed-off-by: Kyle Fazzari <kyrofa@ubuntu.com>

.gitignore

@@ -2,3 +2,4 @@ parts/
 stage/
 prime/
 *.snap
+snap/.snapcraft/

snap/snapcraft.yaml

@@ -6,6 +6,8 @@ description: |
   developer. A virtual lighttable and darkroom for photographers. It manages
   your digital negatives in a database, lets you view them through a zoomable
   lighttable and enables you to develop raw images and enhance them.
+
+grade: stable
 confinement: strict
 
 apps:
@@ -37,6 +39,11 @@ parts:
       - libjson-glib-dev
       - libexiv2-dev
       - libpugixml-dev
+
+    # Stage the lensfun database used for lens correction
+    stage-packages:
+      - liblensfun0
+
     after: [desktop-gtk3]
 
   desktop-gtk3: