app/core: Declare libarchive dependency introduced by 6597dabe

Otherwise, the '-I' param will not be set and Apple Clang will
fail on exotic environments like Homebrew on
which not everything is in the common 'includedir'.

app/core/meson.build

@@ -277,6 +277,7 @@ libappcore = static_library('appcore',
     cairo,
     gegl,
     gdk_pixbuf,
+    libarchive,
     libmypaint,
     gexiv2,
     appstream,