fs/Charset: GetFSCharset() returns "utf-8" by default

If fs_charset is empty, i.e. we're using the default "utf-8",
GetFSCharset() should return exactly that instead of an empty
std::string.

src/fs/Charset.hxx

@@ -29,7 +29,7 @@
  * Gets file system character set name.
  */
 gcc_const
-const std::string &
+const char *
 GetFSCharset();
 
 void