[gtk-doc] mkdb: don't uses sys.exit in library

[Stefan Sauer <stefkost src gnome org>]

commit 8ad03e164f3091df68a9fed5492b54cd61d0ac51
Author: Stefan Sauer <ensonic users sf net>
Date:   Thu Oct 26 22:16:57 2017 +0200

    mkdb: don't uses sys.exit in library
    
    Just print a warning and go on.

 gtkdoc/mkdb.py |    4 ++--
 1 files changed, 2 insertions(+), 2 deletions(-)
---
diff --git a/gtkdoc/mkdb.py b/gtkdoc/mkdb.py
index 3b0bc15..d89d078 100644
--- a/gtkdoc/mkdb.py
+++ b/gtkdoc/mkdb.py
@@ -31,7 +31,6 @@ import logging
 import os
 import re
 import string
-import sys
 
 from . import common, md_to_db
 
@@ -1123,7 +1122,8 @@ def OutputDeclaration(symbol, declaration):
     elif dtype == 'USER_FUNCTION':
         return OutputFunction(symbol, declaration, dtype)
     else:
-        sys.exit("Unknown symbol type " + dtype)
+        logging.warning("Unknown symbol type %s for symbol %s", dtype, symbol)
+        return ('', '')
 
 
 def OutputSymbolTraits(symbol):