This allows the `g_free()` wrapper introduced in the previous commit to
only be defined if `free_sized()` is actually available to improve
performance.

This avoids passing an allocation size to every `g_free()` call if it’s
not going to be used, saving a register store instruction each time.

Suggested by Marco Trevisan in
!3252 (comment 1660032)



Signed-off-by: Philip Withnall <pwithnall@endlessos.org>