Added rcheevos to Externals

Adds the rcheevos library from RetroAchievements to the Dolphin Externals as a submodule. Change was verified to import correctly and build both via Visual Studio and via cmake ninja.

Source/Core/Core/CMakeLists.txt

@@ -602,6 +602,7 @@ PUBLIC
   ${MBEDTLS_LIBRARIES}
   pugixml
   RangeSet::RangeSet
+	rcheevos
   sfml-network
   sfml-system
   videonull

Source/Core/DolphinLib.vcxproj

@@ -52,6 +52,7 @@
   <Import Project="$(ExternalsDir)minizip\exports.props" />
   <Import Project="$(ExternalsDir)picojson\exports.props" />
   <Import Project="$(ExternalsDir)pugixml\exports.props" />
+  <Import Project="$(ExternalsDir)rcheevos\exports.props" />
   <Import Project="$(ExternalsDir)SDL\exports.props" />
   <Import Project="$(ExternalsDir)SFML\exports.props" />
   <Import Project="$(ExternalsDir)soundtouch\exports.props" />

Source/Core/DolphinQt/DolphinQt.vcxproj

@@ -436,6 +436,7 @@
   <Import Project="$(ExternalsDir)mbedtls\exports.props" />
   <Import Project="$(ExternalsDir)mGBA\exports.props" />
   <Import Project="$(ExternalsDir)picojson\exports.props" />
+  <Import Project="$(ExternalsDir)rcheevos\exports.props" />
   <Import Project="$(ExternalsDir)SFML\exports.props" />
   <Import Project="$(ExternalsDir)soundtouch\exports.props" />
   <Import Project="$(ExternalsDir)zstd\exports.props" />