[OE-core] Incomplete runtime dependencies for gdb python PACKAGECONFIG

[Andre McCurdy]

The runtime dependencies for gdb seem to be incomplete when python
support is enabled via gdb's PACKAGECONFIG.

gdb ends up with a runtime dependency on libpython, but not on python
or any python modules, and gdb then fails to run:

  root at 7401:~# gdb
  Could not find platform independent libraries <prefix>
  Could not find platform dependent libraries <exec_prefix>
  Consider setting $PYTHONHOME to <prefix>[:<exec_prefix>]
  ImportError: No module named site

Manually installing python and all python modules seems to be a fix,
but I haven't narrowed down which specific python modules are
required.

The issue is a little more serious for anyone using meta-qt5, since
meta-qt5 (fido and later) contains a .bbappend for gdb which enables
the python PACKAGECONFIG by default:

  https://github.com/meta-qt5/meta-qt5/commit/9d17546d56b5e710a6420b482514202236f410e3