Add support for pkgconfig

[JonasVautherin]

This adds support for pkgconfig. Closes #1428.

The way to get the library in cmake using pkgconfig is:

find_package(PkgConfig REQUIRED)
pkg_check_modules(MAVSDK REQUIRED mavsdk)

target_link_libraries(gimbal
    ${MAVSDK_LIBRARIES}
)

target_include_directories(gimbal
    PRIVATE ${MAVSDK_INCLUDE_DIRS}
)

[julianoes]

I've tested this, it's good to be merged.