[premake] Formatting, cleanup

third_party/imgui.lua

@@ -7,15 +7,9 @@ project("imgui")
     "imgui",
   })
   files({
-    "imgui/imconfig.h",
     "imgui/imgui.cpp",
-    "imgui/imgui.h",
     "imgui/imgui_demo.cpp",
     "imgui/imgui_draw.cpp",
-    "imgui/imgui_internal.h",
     "imgui/imgui_tables.cpp",
     "imgui/imgui_widgets.cpp",
-    "imgui/imstb_rectpack.h",
-    "imgui/imstb_textedit.h",
-    "imgui/imstb_truetype.h",
   })