[gdb] Add & in catch in svr4_handle_solib_event

In svr4_handle_solib_event I noticed:
...
	catch (const gdb_exception_error)
...

This seems to be the only place were we do this, elsewhere we have:
...
	catch (const gdb_exception_error &)
...

I suppose the intent of adding '&' is to avoid a copy.  I'm not sure if it's
necessary given that it's an unnamed const parameter, but I suppose it can't
hurt either.

Add the '&' here as well.

Tested on x86_64-linux.

Approved-By: Tom Tromey <tom@tromey.com>