Fails to build on Linux/BSDs

[jbeich]

0091464 removed slowStartup-16.png but after c8d6f95 the file is referenced again.

$ ./mach build
[...]
17:02.63 Traceback (most recent call last):
17:02.63   File "/usr/local/lib/python2.7/runpy.py", line 174, in _run_module_as_main
17:02.63     "__main__", fname, loader, pkg_name)
17:02.63   File "/usr/local/lib/python2.7/runpy.py", line 72, in _run_code
17:02.63     exec code in run_globals
17:02.63   File "python/mozbuild/mozbuild/action/jar_maker.py", line 17, in <module>
17:02.63     sys.exit(main(sys.argv[1:]))
17:02.63   File "python/mozbuild/mozbuild/action/jar_maker.py", line 13, in main
17:02.63     return mozbuild.jar.main(args)
17:02.63   File "python/mozbuild/mozbuild/jar.py", line 597, in main
17:02.63     jm.makeJar(infile, options.d)
17:02.63   File "python/mozbuild/mozbuild/jar.py", line 333, in makeJar
17:02.63     self.processJarSection(info, jardir)
17:02.63   File "python/mozbuild/mozbuild/jar.py", line 388, in processJarSection
17:02.63     self._processEntryLine(e, outHelper, jf)
17:02.63   File "python/mozbuild/mozbuild/jar.py", line 449, in _processEntryLine
17:02.63     ', '.join(src_base)))
17:02.63 RuntimeError: File "slowStartup-16.png" not found in browser/themes/linux, objdir/browser/themes/linux

https://github.com/MrAlex94/Waterfox/blob/394096b8a96f/browser/themes/linux/jar.mn#L31