Add external tool on Guix for dumppdf.

Strictly speaking, the package in Guiux ships dumppdf.py, but this
should reduce the downstream diff.

diffoscope/external_tools.py

@@ -35,7 +35,7 @@ EXTERNAL_TOOLS = {
         "FreeBSD": "libarchive",
         "guix": "libarchive",
     },
-    "dumppdf": {"debian": "python3-pdfminer"},
+    "dumppdf": {"debian": "python3-pdfminer", "guix": "python-pdfminer-six"},
     "bzip2": {"debian": "bzip2", "arch": "bzip2", "guix": "bzip2"},
     "cbfstool": {},
     "cd-iccdump": {