gitignore: add missing demux_generic.9 entry

The demux_generic component added in #3906 missed adding its
generated man page to docs/man/.gitignore.

Author: Luca Toniolo

docs/man/.gitignore

@@ -312,6 +312,7 @@ man9/ddt.9
 man9/deadzone.9
 man9/debounce.9
 man9/demux.9
+man9/demux_generic.9
 man9/differential.9
 man9/div2.9
 man9/edge.9