cmake: Fix CMake package files not being installed on FreeBSD

By default libraries and their CMake package files should be installed
on Linux and FreeBSD and omitted on Windows. This explicitly adds
FreeBSD.

cmake/Modules/ObsHelpers.cmake

@@ -238,7 +238,7 @@ endfunction()
 function(export_target target)
   set(CMAKE_EXPORT_PACKAGE_REGISTRY OFF)
 
-  if(OS_LINUX)
+  if(OS_LINUX OR OS_FREEBSD)
     set(_EXCLUDE "")
   else()
     set(_EXCLUDE "EXCLUDE_FROM_ALL")