Add missing include guard to avoid declaring ImFontAtlasGetBuilderForStbTruetype() function.

https://github.com/cimgui/cimgui/issues/193
https://github.com/imgui-rs/imgui-rs/pull/582

imgui.h

@@ -64,7 +64,7 @@ Index of this file:
 // Version
 // (Integer encoded as XYYZZ for use in #if preprocessor conditionals. Work in progress versions typically starts at XYY99 then bounce up to XYY00, XYY01 etc. when release tagging happens)
 #define IMGUI_VERSION               "1.86"
-#define IMGUI_VERSION_NUM           18600
+#define IMGUI_VERSION_NUM           18601
 #define IMGUI_CHECKVERSION()        ImGui::DebugCheckVersionAndDataLayout(IMGUI_VERSION, sizeof(ImGuiIO), sizeof(ImGuiStyle), sizeof(ImVec2), sizeof(ImVec4), sizeof(ImDrawVert), sizeof(ImDrawIdx))
 #define IMGUI_HAS_TABLE
 

imgui_internal.h

@@ -2815,7 +2815,9 @@ struct ImFontBuilderIO
 };
 
 // Helper for font builder
+#ifdef IMGUI_ENABLE_STB_TRUETYPE
 IMGUI_API const ImFontBuilderIO* ImFontAtlasGetBuilderForStbTruetype();
+#endif
 IMGUI_API void      ImFontAtlasBuildInit(ImFontAtlas* atlas);
 IMGUI_API void      ImFontAtlasBuildSetupFont(ImFontAtlas* atlas, ImFont* font, ImFontConfig* font_config, float ascent, float descent);
 IMGUI_API void      ImFontAtlasBuildPackCustomRects(ImFontAtlas* atlas, void* stbrp_context_opaque);