Fix compiler warnings

These two issues prevented compilation on my machine:

* unused variable in src/libOpenImageIO/imagebufalgo.cpp:
```
[ 15%] Building CXX object src/libOpenImageIO/CMakeFiles/OpenImageIO.dir/imagebufalgo.cpp.o
/home/mariusz/myrepos/oiio/src/libOpenImageIO/imagebufalgo.cpp: In instantiation of ‘bool OpenImageIO::v1_4::resize_(OpenImageIO::v1_4::ImageBuf&, const OpenImageIO::v1_4::ImageBuf&, OpenImageIO::v1_4::Filter2D*, OpenImageIO::v1_4::ROI, int) [with DSTTYPE = float; SRCTYPE = float]’:
/home/mariusz/myrepos/oiio/src/libOpenImageIO/imagebufalgo.cpp:545:5:   required from here
/home/mariusz/myrepos/oiio/src/libOpenImageIO/imagebufalgo.cpp:372:12: error: variable ‘column’ set but not used [-Werror=unused-but-set-variable]
     float *column = NULL;
            ^
```

* missing include in src/include/strided_ptr.h:
```
[  9%] Building CXX object src/libutil/CMakeFiles/array_view_test.dir/array_view_test.cpp.o
In file included from /home/mariusz/myrepos/oiio/src/libutil/array_view_test.cpp:32:0:
/home/mariusz/myrepos/oiio/src/include/strided_ptr.h:52:31: error: ‘ptrdiff_t’ has not been declared
     strided_ptr (T* ptr=NULL, ptrdiff_t stride=sizeof(T))
```