meshtasticd: add python-pyelftools to host build deps

meshtasticd: add python-pyelftools to host build deps

PlatformIO's SCons builder loads its tool modules at the start of
'pio run', including platformio/builder/tools/piosize.py which imports
elftools.elf. python-platformio's HOST_BUILD_DEPENDS already lists
python-pyelftools/host so it is installed alongside platformio in
staging_dir/hostpkg, but the dependency does not always reach
meshtasticd's compile step via that indirect chain, leading to:

  ModuleNotFoundError: No module named 'elftools'
    File ".../site-packages/platformio/builder/tools/piosize.py", line 22:
      from elftools.elf.descriptions import describe_sh_flags

Declare python-pyelftools/host directly in meshtasticd's build deps so
the host install is guaranteed before 'pio run' is invoked.

Signed-off-by: Alexandru Ardelean <redacted>