Output warnings about skipping filename to stderr

Considering that the output of this script is typically redirected, it
really doesn't make sense to send error messages to the generated .c
file.

misc/scripts/png2c.py

@@ -37,7 +37,7 @@ for path in sys.argv[1:]:
 
         # Allow only filenames that make sense as C variable names
         if not(m):
-                print("Skipped file (unsuitable filename): " + filename)
+                print("Skipped file (unsuitable filename): " + filename, file=sys.stderr)
                 continue
 
         # Read PNG file as character array