ignore untracked files generated during the build

Ignore the following untracked files which are generated during the MSVC build:
```
# HEAD detached at 924bcb7
# Untracked files:
#   (use "git add <file>..." to include in what will be committed)
#
#       src/image/Makefile.Debug
#       src/image/Makefile.Release
#       src/image/wkhtmltoimage_resource.rc
#       src/lib/Makefile.Debug
#       src/lib/Makefile.Release
#       src/lib/wkhtmltox_resource.rc
#       src/pdf/Makefile.Debug
#       src/pdf/Makefile.Release
#       src/pdf/wkhtmltopdf_resource.rc
```