Add `xmlns` attribute in written gpx for better garmin compatibility

[OttoBos]

I noted that the exported GPX files are not accepted by Garmin software (for example Basecamp), and found out this is due to a missing namespace declaration. This PR adds the necessary xmlns tag to the gpx element.

• [X] I agree to follow the project's code of conduct.

• [X] I added an entry to CHANGELOG.md if knowledge of this change could be valuable to users.

[urschrei]

bors try

[bors[bot]]

try

Build failed:

• ci result

[urschrei]

CI failure caused by Time's MSRV of 1.53. Hopefully fixed in https://github.com/georust/gpx/pull/68 when it merges.

[urschrei]

bors try

[bors[bot]]

try

Build succeeded:

• ci result

[urschrei]

bors r+

[urschrei]

Thanks for your contribution @OttoBos!

[bors[bot]]

Build succeeded:

• ci result