standardize imgui headers as user instead of system

src/app/gfx/snes_palette.cc

@@ -1,7 +1,7 @@
 #include "snes_palette.h"
 
 #include <SDL.h>
-#include <imgui/imgui.h>
+#include "imgui/imgui.h"
 
 #include <cstdint>
 #include <cstdlib>
@@ -389,4 +389,4 @@ absl::Status LoadAllPalettes(const Bytes& rom_data, PaletteGroupMap& groups) {
 
 }  // namespace gfx
 }  // namespace app
-}  // namespace yaze
+}  // namespace yaze