Fix a small bug in gdb.rust/simple.rs

Fix a small bug in gdb.rust/simple.rs

I noticed that gdb.rust/simple.rs had two local variables named "v".
This didn't previous cause problems, but with a newer rust compiler
this resulted in a test failure.  (It should have failed all along, so
I suppose earlier passes were due to a compiler bug.)

This patch renames the second variable.

gdb/testsuite/ChangeLog
2018-08-31  Tom Tromey  <tom@tromey.com>

	* gdb.rust/simple.rs: Rename second variable "v".