linux: fix error with IR remotes

Support for IR remotes was originally introduced in NixOS in 2013 with
[1]. This worked fine until 2018 with the release of Linux 4.16 which
contained [2], which removed the default-enable on the IR decoders.
This means that kernels since then build with RC_DEVICES enabled, but
RC_DECODERS disabled:
```
CONFIG_RC_DEVICES=y
```

This breaks IR remote support and also leads to the following error on
every bootup when such a device is present as devices have a default
keymap which uses a protocols decoder, but these decoders are not
available:
```
rc_core: Loaded IR protocol module ir-rc6-decoder, but protocol rc-6 still not available
```

Fix this by also enabling RC_DECODERS in the kernel configuration.

[1] b7ccfc258a
[2] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=22756ae7319b0afc2a80fbdec365a6976a1ad350

pkgs/os-specific/linux/kernel/common-config.nix

@@ -896,6 +896,7 @@ let
 
       REGULATOR  = yes; # Voltage and Current Regulator Support
       RC_DEVICES = option yes; # Enable IR devices
+      RC_DECODERS = option yes; # Required for IR devices to work
 
       RT2800USB_RT53XX = yes;
       RT2800USB_RT55XX = yes;