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/hw/rsa/rsa.cpp

@@ -52,8 +52,8 @@ void InitSlots() {
         return;
     initialized = true;
 
-    const std::string filepath = FileUtil::GetUserPath(FileUtil::UserPath::SysDataDir) + BOOTROM9;
-    FileUtil::IOFile file(filepath, "rb");
+    const std::string filepath = Common::FS::GetUserPath(Common::FS::UserPath::SysDataDir) + BOOTROM9;
+    Common::FS::IOFile file(filepath, "rb");
     if (!file) {
         return;
     }