Ignore mage_output_file.go in mage watch config

[toddtreece]

What this PR does / why we need it:

@yesoreyeram reported that the generation of mage_output_file.go was causing an infinite loop:

[Bra] 11-18 12:58:55 [DEBUG] Running: mage [-v reloadPlugin]
[Bra] 11-18 12:58:55 [ INFO] "$WORKDIR/mage_output_file.go": CREATE
[Bra] 11-18 12:58:55 [DEBUG] Running: mage [-v build:backend]

my best guess is that the plugin build is taking longer than build_delay for @yesoreyeram's plugin, and in my case it's completing and removing mage_output_file.go before the build_delay expires.

adding mage_output_file.go to the ignore_files list seems to fix this issue

Which issue(s) this PR fixes:

Fixes #

Special notes for your reviewer:

[marefr]

LGTM