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="Harold Grovesteen"

 reporter_url="http://sourceware.org/ml/crossgcc/2009-11/msg00052.html"

 reporter_comment=""