Fix build on Windows

Commit cfcc9b63 causes unit test failure on Windows.

    [junit] Testsuite: fitnesse.components.ClassPathBuilderTest
    [junit] Tests run: 1, Failures: 0, Errors: 1, Time elapsed: 0.032 sec
    [junit]
    [junit] Testcase: testGetClasspath took 0.022 sec
    [junit]     Caused an ERROR
    [junit] null
    [junit] java.lang.ExceptionInInitializerError
    [junit]     at fitnesse.components.ClassPathBuilder.getPathSeparator(ClassPathBuilder.java:27)
    [junit]     at fitnesse.components.ClassPathBuilder.getClasspath(ClassPathBuilder.java:23)
    [junit]     at fitnesse.components.ClassPathBuilderTest.testGetClasspath(ClassPathBuilderTest.java:35)
    [junit] Caused by: java.util.regex.PatternSyntaxException: Unexpected internal error near index 1
    [junit] \
    [junit]  ^
    [junit]     at java.util.regex.Pattern.error(Pattern.java:1713)
    [junit]     at java.util.regex.Pattern.compile(Pattern.java:1466)
    [junit]     at java.util.regex.Pattern.<init>(Pattern.java:1133)
    [junit]     at java.util.regex.Pattern.compile(Pattern.java:823)
    [junit]     at java.lang.String.split(String.java:2292)
    [junit]     at java.lang.String.split(String.java:2334)
    [junit]     at fitnesse.responders.run.TestSystem.fitnesseJar(TestSystem.java:32)
    [junit]     at fitnesse.responders.run.TestSystem.<clinit>(TestSystem.java:16)
    [junit]

    BUILD FAILED

This is due to the path separator on Windows being the backslash (\) character and that, on its own, is not a valid regex pattern.