Minor bug fix to autodiff.h

Change-Id: Ib41050a2f2ba1898c71ff19d74f8eca2496212c0

include/ceres/internal/autodiff.h

diff --git a/include/ceres/internal/autodiff.h b/include/ceres/internal/autodiff.h
index cf21d7a..3a96625 100644
--- a/include/ceres/internal/autodiff.h
+++ b/include/ceres/internal/autodiff.h
@@ -172,7 +172,7 @@
   for (int j = 0; j < N; ++j) {
     dst[j].a = src[j];
     dst[j].v.setZero();
-    dst[j].v[offset + j] = 1.0;
+    dst[j].v[offset + j] = T(1.0);
   }
 }