warnings: mute unused variable warning

../../../src/viewer/datasource.c: In function ‘mcview_set_byte’:
../../../src/viewer/datasource.c:270:38: error: unused parameter ‘offset’ [-Werror=unused-parameter]
  270 | mcview_set_byte (WView * view, off_t offset, byte b)
      |                                ~~~~~~^~~~~~

Signed-off-by: Yury V. Zaytsev <yury@shurup.com>

src/viewer/datasource.c

@@ -270,6 +270,7 @@ void
 mcview_set_byte (WView *view, off_t offset, byte b)
 {
     (void) &b;
+    (void) offset;
 
     g_assert (offset < mcview_get_filesize (view));
     g_assert (view->datasource == DS_FILE);