Fix compilation error caused by missing suitesparse headers

Covariance implementation file used to unconditionally include
SuiteSparseQR.hpp which caused compilation error in cases you
don't have SuiteSuite installed to the system

Moved the include to #ifdef block.

Change-Id: I3a52c0f81711b2b70ae625fe80b758ecb0817cc6

internal/ceres/covariance_impl.cc

diff --git a/internal/ceres/covariance_impl.cc b/internal/ceres/covariance_impl.cc
index 1befde7..ca5be49 100644
--- a/internal/ceres/covariance_impl.cc
+++ b/internal/ceres/covariance_impl.cc
@@ -48,7 +48,10 @@
 #include "ceres/suitesparse.h"
 #include "ceres/wall_time.h"
 #include "glog/logging.h"
-#include "SuiteSparseQR.hpp"
+
+#ifndef CERES_NO_SUITESPARSE
+#  include "SuiteSparseQR.hpp"
+#endif
 
 namespace ceres {
 namespace internal {