vulkan_common: Move dynamic library load to a separate file

Allows us to initialize a Vulkan dynamic library from different backends
without duplicating code.

src/video_core/CMakeLists.txt

@@ -260,6 +260,8 @@ add_library(video_core STATIC
     textures/texture.h
     video_core.cpp
     video_core.h
+    vulkan_common/vulkan_library.cpp
+    vulkan_common/vulkan_library.h
 )
 
 create_target_directory_groups(video_core)