[gdb/testsuite] Fix gdb.base/jit-bfd-name.exp

When running test-case gdb.base/jit-bfd-name.exp, I run into:
...
ERROR: tcl error sourcing gdb/testsuite/gdb.base/jit-bfd-name.exp.
ERROR: can't read "start": no such variable
...

The problem is that commit c96ceed9dce ("gdb: include the end address in
in-memory bfd filenames") introduced a use of variable start, but not a
definition.

Fix this by adding the missing definition.

Tested on x86_64-linux.