The build logs and the Fedora C99 show that these functions are called without a proper previous declaration:

/builddir/build/BUILD/fcitx-fbterm-0.2.0/src/fcitx-fbterm.c: In function 'im_active':
/builddir/build/BUILD/fcitx-fbterm-0.2.0/src/fcitx-fbterm.c:116:5: warning: implicit declaration of function 'fcitx_client_enable_ic'; did you mean 'fcitx_client_focus_in'? [-Wimplicit-function-declaration]
  116 |     fcitx_client_enable_ic(client);
      |     ^~~~~~~~~~~~~~~~~~~~~~
      |     fcitx_client_focus_in
/builddir/build/BUILD/fcitx-fbterm-0.2.0/src/fcitx-fbterm.c: In function 'im_deactive':
/builddir/build/BUILD/fcitx-fbterm-0.2.0/src/fcitx-fbterm.c:126:5: warning: implicit declaration of function 'fcitx_client_close_ic'; did you mean 'fcitx_client_reset'? [-Wimplicit-function-declaration]
  126 |     fcitx_client_close_ic(client);
      |     ^~~~~~~~~~~~~~~~~~~~~
      |     fcitx_client_reset

I investigated this and it seems the proper fix is actually in fcitx: Declare these functions in the appropriate header file. I submitted a pull request to this effect:

  fcitx-gclient: Export fcitx_client_enable_ic, fcitx_client_close_ic
  <https://github.com/fcitx/fcitx/pull/521>

This bug is just a reminder to re-test fcitx-fbterm once fcitx has been updated accordingly.