sys_util: Remove extra semicolon to fix warning on nightly.

Nightly rust generates errors of the following form:
warning: trailing semicolon in macro used in expression position
   --> /home/allenwebb/external/NoBackup/chromiumos/src/platform/crosvm/common/sys_util/src/syslog.rs:193:10
    |
188 | / macro_rules! lock {
189 | |     () => {
190 | |         match lock() {
191 | |             Ok(s) => s,
192 | |             _ => return,
193 | |         };
    | |          ^
194 | |     };
195 | | }
    | |_- in this expansion of `lock!`
...
336 |       let mut state = lock!();
    |                       ------- in this macro invocation
    |
    = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
    = note: for more information, see issue #79813 <https://github.com/rust-lang/rust/issues/79813>

BUG=None
TEST=cargo test

Change-Id: I323d5a4d88d1e2fe4b7745d3f49adef808ee7016
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/crosvm/+/3318767
Auto-Submit: Allen Webb <allenwebb@google.com>
Commit-Queue: Daniel Verkamp <dverkamp@chromium.org>
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Daniel Verkamp <dverkamp@chromium.org>

common/sys_util/src/syslog.rs

@@ -190,7 +190,7 @@ macro_rules! lock {
         match lock() {
             Ok(s) => s,
             _ => return,
-        };
+        }
     };
 }