segfault in ImFont::CalcTextSizeA

see:
https://github.com/prusa3d/PrusaSlicer/issues/9774

Index: src/slic3r/GUI/GUI_App.cpp
--- src/slic3r/GUI/GUI_App.cpp.orig
+++ src/slic3r/GUI/GUI_App.cpp
@@ -1660,7 +1660,7 @@ bool GUI_App::on_init_inner()
         // and wxEVT_SET_FOCUS before GUI_App::post_init is called) wasn't called before GUI_App::post_init and OpenGL wasn't initialized.
         // Since issue #9774 Where same problem occured on MacOS Ventura, we decided to have this check on MacOS as well.
 
-#if defined(__linux__) || defined(__APPLE__)
+#if defined(__linux__) || defined(__APPLE__) || defined(__OpenBSD__)
         if (!m_post_initialized && m_opengl_initialized) {
 #else
         if (!m_post_initialized) {
