Fix: lttng: -Wshadow error in cmd_snapshot

Fix: lttng: -Wshadow error in cmd_snapshot

The optarg variable name shadows a variable in an external dependency.
clang returns the following error on my machine:
  commands/snapshot.c:627:16: error: declaration shadows a variable in the global scope [-Werror,-Wshadow]
                          const char *optarg = poptGetOptArg(pc);
                                      ^
  /usr/include/x86_64-linux-gnu/bits/getopt_core.h:36:14: note: previous declaration is here
  extern char *optarg;

Signed-off-by: Francis Deslauriers <francis.deslauriers@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Change-Id: I85dd7b0761ebc2c023d84ba869c0551f91f38a17