mappings: handle separation of dtrace

Fedora decided to separate dtrace from sdt headers: https://fedoraproject.org/wiki/Changes/Separate_dtrace_package

Similarly, OpenSUSE has a separate package in Tumbleweed: https://software.opensuse.org/package/systemtap-dtrace

Signed-off-by: Ján Tomko jtomko@redhat.com