cmake: Fix CMake (v3.16) cannot create ALIAS target "Wpe::FDO" error

[psaavedra]

... because target "PkgConfig::WpeFDO" is imported but not globally visible in cmake/FindWpeFDO.cmake.

Notice that this issue is not reproducible in CMake v3.22.

CMake doc [1][2] says:

The scope of the definition of an IMPORTED target is the directory
where it was defined. It may be accessed and used from
subdirectories, but not from parent directories or sibling directories.
The scope is similar to the scope of a cmake variable.

It is also possible to define a GLOBAL IMPORTED target which is
accessible globally in the buildsystem.

[1] 3.16: https://cmake.org/cmake/help/v3.16/manual/cmake-buildsystem.7.html#imported-targets
[2] latest: https://cmake.org/cmake/help/latest/manual/cmake-buildsystem.7.html#imported-targets

Note: Target branch is cog-0.14 since there is not longer CMake rules in the master branch.

[psaavedra]

Error:

| CMake Error at cmake/FindWpeFDO.cmake:5 (add_library):
|   add_library cannot create ALIAS target "Wpe::FDO" because target
|   "PkgConfig::WpeFDO" is imported but not globally visible.
| Call Stack (most recent call first):
|   platform/wayland/CMakeLists.txt:28 (find_package)
| 
| 
| -- Found WpeFDO: WPEBackend-fdo-1.0;wpe-1.0;xkbcommon (found version "1.12.0")
| -- Checking for module 'wayland-egl'
| --   Found wayland-egl, version 18.1.0
| -- Checking for module 'wayland-cursor'
| --   Found wayland-cursor, version 1.18.0
| -- Checking for module 'epoxy'
| --   Found epoxy, version 1.5.4
| -- Checking for module 'libdrm>=2.4.71'
| --   Found libdrm, version 2.4.101
| -- Checking for module 'gbm>=13.0'
| --   Found gbm, version 20.0.2
| -- Checking for module 'libinput'
| --   Found libinput, version 1.15.2
| -- Checking for module 'libudev'
| --   Found libudev, version 243
| -- Checking for module 'wayland-server'
| --   Found wayland-server, version 1.18.0
| CMake Error at cmake/FindWpeFDO.cmake:5 (add_library):
|   add_library cannot create ALIAS target "Wpe::FDO" because target
|   "PkgConfig::WpeFDO" is imported but not globally visible.
| Call Stack (most recent call first):
|   platform/drm/CMakeLists.txt:9 (find_package)
| 
| 
| -- Configuring incomplete, errors occurred!

[psaavedra]

@aperezdc, I'm not sure how to proceed. The https://github.com/Igalia/cog/runs/8203858606?check_suite_focus=true blames against a changed not done in this patch.

[aperezdc]

Good find, thanks @psaavedra! I'll merge this manually, the style checker is being silly.