obs-ffmpeg: Move external headers to external dir

Prevents automatic formatting with clang-format

plugins/obs-ffmpeg/external/.clang-format

@@ -0,0 +1,3 @@
+Language: Cpp
+SortIncludes: false
+DisableFormat: true

plugins/obs-ffmpeg/jim-nvenc.h

@@ -4,7 +4,7 @@
 #include <windows.h>
 
 #include <obs-module.h>
-#include "nvEncodeAPI.h"
+#include "external/nvEncodeAPI.h"
 
 typedef NVENCSTATUS(NVENCAPI *NV_CREATE_INSTANCE_FUNC)(
 	NV_ENCODE_API_FUNCTION_LIST *);