wl_screenshot: Fix eina_file_mkstemp usage.
wl_screenshot: Fix eina_file_mkstemp usage.

In my previous commit I did not took into account that ein_file_mkstemp
does not alter the given filename. This is different from mkstemp and I
missed that.

We now ask for the new filename and use it instead.

rvandegrift/e,tizenorg/platform.upstream.enlightenment,tizenorg/platform.upstream.enlightenment,tasn/enlightenment,tizenorg/platform.upstream.enlightenment,tasn/enlightenment,rvandegrift/e,FlorentRevest/Enlightenment,FlorentRevest/Enlightenment,rvandegrift/e,FlorentRevest/Enlightenment,tasn/enlightenment