Revert "gdbsupport: remove unnecessary `#ifndef IN_PROCESS_AGENT`"

This reverts commit fe72c327.

It turns out it was wrong, libinproctrace.so does build its own
gdbsupport/tdesc.cc.  This broke the build:

    make[1]: Entering directory '/home/simark/build/binutils-gdb-one-target/gdbserver'
      CXXLD  libinproctrace.so
    /usr/bin/ld: gdbsupport/tdesc-ipa.o: in function `print_xml_feature::visit_pre(target_desc const*)':
    /home/simark/src/binutils-gdb/gdbserver/../gdbsupport/tdesc.cc:407: undefined reference to `tdesc_architecture_name(target_desc const*)'
    /usr/bin/ld: /home/simark/src/binutils-gdb/gdbserver/../gdbsupport/tdesc.cc:408: undefined reference to `tdesc_architecture_name(target_desc const*)'
    /usr/bin/ld: /home/simark/src/binutils-gdb/gdbserver/../gdbsupport/tdesc.cc:411: undefined reference to `tdesc_osabi_name(target_desc const*)'
    /usr/bin/ld: /home/simark/src/binutils-gdb/gdbserver/../gdbsupport/tdesc.cc:416: undefined reference to `tdesc_compatible_info_list(target_desc const*)'
    /usr/bin/ld: /home/simark/src/binutils-gdb/gdbserver/../gdbsupport/tdesc.cc:418: undefined reference to `tdesc_compatible_info_arch_name(std::unique_ptr<tdesc_compatible_info, std::default_delete<tdesc_compatible_info> > const&)'