[v1.4] cmake: copy correct mavlink directory

[julianoes]

Hopefully, this time I get it right.

[eastman-bechamo]

This fixed the header install issue. Thanks a bunch!

[Katawann]

works for me ! Thank you @julianoes

[Consti10]

I independently came across this issue in our mavlink fork and ended up doing the same fix implemented here

aka use ${MAVLINK_HEADERS}/mavlink instead of ${MAVLINK_HEADERS}