Hide dsyrk_ if CERES_NO_LAPACK is defined.

This will allow iOS users not to suffer Apple's
idiotic AppStore approval process.

Change-Id: I34ad3f458b0cf8ffb0f89205937233264c3bb63a

internal/ceres/blas.cc

diff --git a/internal/ceres/blas.cc b/internal/ceres/blas.cc
index b919e13..da49183 100644
--- a/internal/ceres/blas.cc
+++ b/internal/ceres/blas.cc
@@ -32,6 +32,7 @@
 #include "ceres/internal/port.h"
 #include "glog/logging.h"
 
+#ifdef CERES_NO_LAPACK
 extern "C" void dsyrk_(char* uplo,
                        char* trans,
                        int* n,
@@ -42,6 +43,7 @@
                        double* beta,
                        double* c,
                        int* ldc);
+#endif
 
 namespace ceres {
 namespace internal {