Add missing return statement.

Change-Id: I5eaf718318e27040e3c97e32ee46cf0a11176a37

internal/ceres/program.cc

diff --git a/internal/ceres/program.cc b/internal/ceres/program.cc
index 04248e7..82d76d3 100644
--- a/internal/ceres/program.cc
+++ b/internal/ceres/program.cc
@@ -225,6 +225,7 @@
     ret += StringPrintf("%d: %s\n",
                         i, parameter_blocks_[i]->ToString().c_str());
   }
+  return ret;
 }
 
 }  // namespace internal