build(jpegenc): add stb as a submodule

.gitmodules

@@ -22,3 +22,6 @@
 [submodule "3rdparty/freetype"]
 	path = 3rdparty/freetype
 	url = https://github.com/freetype/freetype.git
+[submodule "3rdparty/stb"]
+	path = 3rdparty/stb
+	url = https://github.com/nothings/stb.git

core/libs/prx/libSceJpegEnc/CMakeLists.txt

@@ -0,0 +1,5 @@
+add_library(libSceJpegEnc SHARED EXCLUDE_FROM_ALL ${CMAKE_CURRENT_SOURCE_DIR}/Export.cpp)
+target_include_directories(libSceJpegEnc PRIVATE ${LIBS_INCLUDE_DIR})
+target_include_directories(libSceJpegEnc SYSTEM PRIVATE ${CMAKE_SOURCE_DIR}/3rdparty/stb)
+set_target_properties(libSceJpegEnc PROPERTIES CXX_EXTENSIONS OFF)
+configure_windows_unwind(libSceJpegEnc)