externals: Track upstream opus

Tracks upstream opus, allowing the library to be easily updated. While
we're at it, we incorporate the CMakeLists.txt so that we have easy
control over the requirements of the build.

.gitmodules

@@ -26,11 +26,11 @@
     path = externals/mbedtls
     url = https://github.com/DarkLordZach/mbedtls
 [submodule "opus"]
-    path = externals/opus
-    url = https://github.com/ogniK5377/opus.git
+    path = externals/opus/opus
+    url = https://github.com/xiph/opus.git
 [submodule "soundtouch"]
-	path = externals/soundtouch
-	url = https://github.com/citra-emu/ext-soundtouch.git
+    path = externals/soundtouch
+    url = https://github.com/citra-emu/ext-soundtouch.git
 [submodule "libressl"]
     path = externals/libressl
     url = https://github.com/citra-emu/ext-libressl-portable.git