service/nvdrv: Take std::string in Open() by const reference

Avoids copies from being made, since the string is only ever used for
lookup, the data is never transfered anywhere.

Ideally, we'd use a std::string_view here, but devices is a
std::unordered_map, not a std::map, so we can't use heterogenous lookup
here.

src/core/hle/service/nvdrv/nvdrv.h

@@ -38,7 +38,7 @@ public:
     }
 
     /// Opens a device node and returns a file descriptor to it.
-    u32 Open(std::string device_name);
+    u32 Open(const std::string& device_name);
     /// Sends an ioctl command to the specified file descriptor.
     u32 Ioctl(u32 fd, u32 command, const std::vector<u8>& input, std::vector<u8>& output);
     /// Closes a device file descriptor and returns operation success.