gdb: ensure extension_language_python is always defined

gdb: ensure extension_language_python is always defined

In this commit:

  commit c6a6aad52d9e839d6a84ac31cabe2b7e1a2a31a0
  Date:   Mon Oct 25 17:25:45 2021 +0100

      gdb/python: make some global variables static

building without Python was broken.  The extension_language_python
global was moved from being always defined, to only being defined when
the HAVE_PYTHON macro was defined.  As a consequence, building without
Python support would result in errors like:

  /usr/bin/ld: extension.o:(.rodata+0x120): undefined reference to `extension_language_python'

This commit fixes the problem by moving the definition of
extension_language_python outside of the HAVE_PYTHON macro protection.