Add serializers/src/**/*.js to .gitignore

After building the file serializers/src/OBJ/babylon.objSerializer.js was
present as an untracked file in the 'git status' output. This change
has git ignore .js files in the serializers/src/ subdirectories.

.gitignore

@@ -23,6 +23,7 @@ proceduralTexturesLibrary/src/**/*.js
 postProcessLibrary/src/**/*.js
 canvas2D/src/**/*.js
 inspector/src/**/*.js
+serializers/src/**/*.js
 
 # Build results
 [Dd]ebug/