psensor: add -Wno-error due to increasing compiler strictness

This Hydra failure:

  https://hydra.nixos.org/log/iq8zqmw7yclgwpvr8i8clwf2sh67jb0i-psensor-1.2.1.drv

...appears to be the result of the package shipping with -Werror and
gcc accreting ever-increasing quantities of warnings as time
progresses.  Let's not treat the compiler warnings as errors.

pkgs/tools/system/psensor/default.nix

@@ -19,7 +19,7 @@ stdenv.mkDerivation rec {
   ];
 
   preConfigure = ''
-    NIX_CFLAGS_COMPILE="$NIX_CFLAGS_COMPILE -I${libXNVCtrl}/include"
+    NIX_CFLAGS_COMPILE="$NIX_CFLAGS_COMPILE -I${libXNVCtrl}/include -Wno-error"
     NIX_LDFLAGS="$NIX_LDFLAGS -L${libXNVCtrl}/lib"
   '';