debug/gdb: don;t install sample gdbinit for old versions

Only starting with gdb-7 does installing the gdbinit sample makes senses.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@anciens.enib.fr>

 reporter_name="Bart vdr. Meulen <bartvdrmeulen@gmail.com>"

 reporter_url=""

 reporter_comment="Config to build cross-compiler with mingw as kernel type"