[mailmap] Add entry for the GN LLVM syncbot

The bot did its first 43 commits under the name "GN Syncbot" before
it switched to "LLVM GN Syncbot". Use the latter as canonical name.

.mailmap

@@ -31,5 +31,6 @@
 Jon Roelofs <jonathan_roelofs@apple.com> Jon Roelofs <jroelofs@jroelofs.com>
 Jon Roelofs <jonathan_roelofs@apple.com> Jonathan Roelofs <jonathan@codesourcery.com>
 Jon Roelofs <jonathan_roelofs@apple.com> Jonathan Roelofs <jroelofs@jroelofs.com>
+LLVM GN Syncbot <llvmgnsyncbot@gmail.com>
 Martin Storsjö <martin@martin.st>
 Saleem Abdulrasool <compnerd@compnerd.org> <compnerd@compnerd.org>