common/fileutil: Convert namespace to Common::FS

Migrates a remaining common file over to the Common namespace, making it
consistent with the rest of common files.

This also allows for high-traffic FS related code to alias the
filesystem function namespace as

namespace FS = Common::FS;

for more concise typing.

src/core/hle/service/apt/apt.cpp

@@ -213,10 +213,10 @@ bool Module::LoadLegacySharedFont() {
     // generated by the APT:U service. The best way to get is by dumping it from RAM. We've provided
     // a homebrew app to do this: https://github.com/citra-emu/3dsutils. Put the resulting file
     // "shared_font.bin" in the Citra "sysdata" directory.
-    std::string filepath = FileUtil::GetUserPath(FileUtil::UserPath::SysDataDir) + SHARED_FONT;
+    std::string filepath = Common::FS::GetUserPath(Common::FS::UserPath::SysDataDir) + SHARED_FONT;
 
-    FileUtil::CreateFullPath(filepath); // Create path if not already created
-    FileUtil::IOFile file(filepath, "rb");
+    Common::FS::CreateFullPath(filepath); // Create path if not already created
+    Common::FS::IOFile file(filepath, "rb");
     if (file.IsOpen()) {
         file.ReadBytes(shared_font_mem->GetPointer(), file.GetSize());
         return true;