gitignore: Add "makefile" to global .gitignore file.

dpgeorge · dpgeorge · commit 49afa5b701d7 · 2016-12-19T15:06:52.000+11:00
"makefile" is used similarly to "GNUmakefile", to override any default
settings in the main "Makefile".
diff --git a/.gitignore b/.gitignore
@@ -37,4 +37,5 @@ __pycache__/
 # Customized Makefile/project overrides
 ######################
 GNUmakefile
+makefile
 user.props
