Fix build failure when pkgconfig is not available

[ckamm]

The cmake step of the build should succeed when the PkgConfig library can't be found. Currently the following error is shown:

 173s] -- Added vfsPlugin without tests: suffix
[  173s] Can't determine libcloudproviders version, not enabling it
[  173s] -- Could NOT find LIBCLOUDPROVIDERS (missing: LIBCLOUDPROVIDERS_LIBRARY LIBCLOUDPROVIDERS_INCLUDE_DIR) 
[  173s] CMake Error at /usr/share/cmake-3.12/Modules/FindPackageHandleStandardArgs.cmake:137 (message):
[  173s]   Could NOT find PkgConfig (missing: PKG_CONFIG_EXECUTABLE)
[  173s] Call Stack (most recent call first):
[  173s]   /usr/share/cmake-3.12/Modules/FindPackageHandleStandardArgs.cmake:378 (_FPHSA_FAILURE_MESSAGE)
[  173s]   /usr/share/cmake-3.12/Modules/FindPkgConfig.cmake:39 (find_package_handle_standard_args)
[  173s]   src/gui/libcloudproviders/libcloudproviders.cmake:2 (find_package)
[  173s]   src/gui/CMakeLists.txt:320 (include)
[  173s] 
[  173s] 
[  173s] -- Configuring incomplete, errors occurred!

[ckamm]

Seen in an automatic build by @guruz in #7235

[HanaGemela]

@ckamm is this something QA should test? If so, please provide steps to recreate

[ckamm]

@HanaGemela No, this issue is only about successfully compiling the client.

[jnweiger]

Linux packages have no support for libcloudproviders currently. If that feature is meant for milestone 2.6.0 then we have a blocker.