libxl: expose #define to 4.5 and above

libxl: expose #define to 4.5 and above

In e3abab74 (libxl: un-constify return value of libxl_basename), the
macro was exposed to releases < 4.5. However only new code is able to
make use of that macro so it should be exposed to releases >= 4.5.

Signed-off-by: Wei Liu <wei.liu2@citrix.com>
Cc: Ian Campbell <ian.campbell@citrix.com>
Cc: Ian Jackson <ian.jackson@eu.citrix.com>
Cc: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>