fs/Traits.hxx: add GetLengthFS()

There is no GetLengthUTF8() because strlen or std::string::size()
could be used instead.

src/fs/Traits.hxx

@@ -29,6 +29,7 @@
 
 #include <string>
 
+#include <string.h>
 #include <assert.h>
 
 /**
@@ -79,6 +80,11 @@ struct PathTraits {
 		return IsSeparatorUTF8(*p);
 	}
 
+	gcc_pure
+	static size_t GetLengthFS(const_pointer p) {
+		return strlen(p);
+	}
+
 	/**
 	 * Determine the "base" file name of the given UTF-8 path.
 	 * The return value points inside the given string.