Fix compilation error on systems without SuiteSparse installed

Issue was caused by declaring class PerThreadContext with some
members of choldmod-specific types. This class in only used from
an #ifndef CERES_NO_SUITESPARSE block and solved compilation error
by simply wrapping PerThreadContext with the same #ifndef block.

Change-Id: Icdc329073fcbd4a328e41ea8c0af0962e9c34ba8

internal/ceres/covariance_impl.cc

@@ -54,6 +54,7 @@ namespace internal {
 namespace {
 
 // Per thread storage for SuiteSparse.
+#ifndef CERES_NO_SUITESPARSE
 struct PerThreadContext {
   explicit PerThreadContext(int num_rows)
       : solution(NULL),
@@ -79,6 +80,7 @@ struct PerThreadContext {
   cholmod_dense* rhs;
   SuiteSparse ss;
 };
+#endif
 
 }  // namespace