Don't lint build output

npm test has been failing if previous build output was present,
because the generated build/msg/??.msg files are missing a trailing
newline and the line to exclude build/* from being linted was
inadvertently omitted from commit 9e72378.

(Much of the rest of the build output would also fail lint checks but
was already excluded by the *_compressed*.js exclusion.)

.eslintignore

@@ -2,6 +2,7 @@
 *_uncompressed*.js
 gulpfile.js
 /msg/*
+/build/*
 /dist/*
 /core/utils/global.js
 /tests/blocks/*