maintainers: add sbellem

For the sgxsdk and ippcrypto packages

maintainers/maintainer-list.nix

···
     githubId = 720864;
     name = "Sébastien Bourdeauducq";
   };
+  sbellem = {
+    email = "[e-mail]";
+    github = "sbellem";
+    githubId = 125458;
+    name = "Sylvain Bellemare";
+  };
   sbond75 = {
     name = "sbond75";
     email = "[e-mail]";