FindBZip2: Require header to be found (#14630)

Add missing DEFAULT_MSG argument to FIND_PACKAGE_HANDLE_STANDARD_ARGS().
Without this, if you only have installed the bzip2 package, but not the
libbz2-dev package, FindBZip2 will report bzip2 as found and provide the
include directory.
Merge topic 'FindBZip2-require-header' into next

a6d073a FindBZip2: Require header to be found (#14630)