Fix use of incomplete type in defaulted Problem methods

- As Problem contains a unique_ptr<ProblemImpl> and as defaulted
  methods are declared inline where ProblemImpl's implementation is not
  available use of '= default' will generate compile errors on strict
  compilers.
- This CL moves the definition of defaulted methods to the
  implementation.

Change-Id: I7d79757cf684378fadef8e6f4000c84387018cdb

include/ceres/problem.h

diff --git a/include/ceres/problem.h b/include/ceres/problem.h
index ba9b903..e43616e 100644
--- a/include/ceres/problem.h
+++ b/include/ceres/problem.h
@@ -189,8 +189,8 @@
   // invocation Problem(Problem::Options()).
   Problem();
   explicit Problem(const Options& options);
-  Problem(Problem&&) = default;
-  Problem& operator=(Problem&&) = default;
+  Problem(Problem&&);
+  Problem& operator=(Problem&&);
 
   Problem(const Problem&) = delete;
   Problem& operator=(const Problem&) = delete;

internal/ceres/problem.cc

diff --git a/internal/ceres/problem.cc b/internal/ceres/problem.cc
index f4efa6e..767fe97 100644
--- a/internal/ceres/problem.cc
+++ b/internal/ceres/problem.cc
@@ -43,6 +43,9 @@
 Problem::Problem() : impl_(new internal::ProblemImpl) {}
 Problem::Problem(const Problem::Options& options)
     : impl_(new internal::ProblemImpl(options)) {}
+// Not inline defaulted in declaration due to use of std::unique_ptr.
+Problem::Problem(Problem&&) = default;
+Problem& Problem::operator=(Problem&&) = default;
 Problem::~Problem() {}
 
 ResidualBlockId Problem::AddResidualBlock(