build: avoid new chmod.c warnings from upcoming GCC12

build: avoid new chmod.c warnings from upcoming GCC12

Here are the warnings:
src/chmod.c:175:3: error: 'ch.new_mode' may be used uninitialized in\
    this function [-Werror=maybe-uninitialized]
  175 |   strmode (ch->new_mode, perms);
src/chmod.c:178:3: error: 'ch.old_mode' may be used uninitialized in\
    this function [-Werror=maybe-uninitialized]
  178 |   strmode (ch->old_mode, old_perms);

* src/chmod.c (process_file): Initialize ch.  Its new_mode and
old_mode fields could indeed be used uninitialized to form mode
strings, but those are used only when built from initialized members.