Fix: sessiond: fix possible buffer overflow warning

Fix: sessiond: fix possible buffer overflow warning

When compiling with clang-14 on Ubuntu 20.04, I get:

      CC       lttng-syscall.lo
    /home/smarchi/src/lttng-tools/src/bin/lttng-sessiond/lttng-syscall.c:70:13: error: 'fscanf' may overflow; destination buffer in argument 4 has size 255, but the corresponding specifier may require size 256 [-Werror,-Wfortify-source]
                                    &index, name, &bitness) == 3) {
                                            ^

I think the compiler is right, we read a string when length up to 255 in
a buffer of size 255.  We need one more byte for the NULL terminator,
fix that.

Change-Id: I6b2eec401af3ef6230dd4b6c8559032de9b54584
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>