smem: enable matplotlib

This makes it able to show a pie chart graph:

smem --pie=pss

I chose this pattern for adding the python package because it seemed simplest to
me.

pkgs/by-name/sm/smem/package.nix

···
     sha256 = "19ibv1byxf2b68186ysrgrhy5shkc5mc69abark1h18yigp3j34m";
   };
 
-  buildInputs = [ python3 ];
+  buildInputs = [
+    (python3.withPackages (pp: [ pp.matplotlib ]))
+  ];
 
   makeFlags = [ "smemcap" ];