Replace include_directories with target_include_directories
Replace include_directories with target_include_directories

Cmake include_directories() does not propagate up, what means that any project
that includes this project as a subdirectory will need to know the exact
location of our headers if it wants to use them. As a result, parent projects
will eventually need to make changes to their include paths if for some reason
we change something in ours, besides having to keep long names in their
\#include directives or include_directories().

For example, the following line would be needed in orther include
mavlink_vehicles.hh in the source code of a parent project:

    ```
    #include "mavlink_vehicles/src/mavlink_vehicles.hh"
    ```

By using target_include_directories(), cmake deals with it transparently when a
target of a parent project links to our library. Then, the source code of that
target would need simply to add the following line to include our headers:

    ```
    #include "mavlink_vehicles.hh"
    ```

Signed-off-by: Guilherme Campos Camargo <564b2b7fead5c6563c25bad017ee4ddcfee1ede9@intel.com>

guiccbr/mavlink-vehicles