Move LibRetro CMake file to new syntax

src/citra_libretro/CMakeLists.txt

 set(CMAKE_MODULE_PATH ${CMAKE_MODULE_PATH} ${PROJECT_SOURCE_DIR}/CMakeModules)
 
-set(SRCS
+add_library(citra_libretro SHARED
         emu_window/libretro_window.cpp
+        emu_window/libretro_window.h
         input/input_factory.cpp
+        input/input_factory.h
         input/mouse_tracker.cpp
+        input/mouse_tracker.h
         citra_libretro.cpp
+        citra_libretro.h
         environment.cpp
+        environment.h
         core_settings.cpp
-        )
-set(HEADERS
-        emu_window/libretro_window.h
-        input/input_factory.h
-        input/mouse_tracker.h
         core_settings.h
-        citra_libretro.h
-        environment.h
         )
 
-create_directory_groups(${SRCS} ${HEADERS})
+create_target_directory_groups(citra_libretro)
 
-add_library(citra_libretro SHARED ${SRCS} ${HEADERS})
 target_link_libraries(citra_libretro PRIVATE common core)
 target_link_libraries(citra_libretro PRIVATE glad libretro)