Include "util.h" in "array.h".

[notoraptor]

It seems "util.h" is not automatically included when we use "array.h". I have encountered the error while trying to use GpuArray_ITEMSIZE(), defined in array.h, which uses gpuarray_get_elsize() defined in util.h.

/home/notoraptor/.theano/compiledir_Linux-4.4--generic-x86_64-with-debian-stretch-sid-x86_64-2.7.13-64/tmpQo9Y8T/mod.cpp: In member function ‘int {anonymous}::__struct_compiled_op_7cd93cb03039f5356757163adef008af::run()’:
/home/notoraptor/.local/include/gpuarray/array.h:197:63: error: ‘gpuarray_get_elsize’ was not declared in this scope
 #define GpuArray_ITEMSIZE(a) gpuarray_get_elsize((a)->typecode)
                                                               ^

This PR add the missing inclusion in array.h.

@abergeron

[nouiz]

I'll wait for @abergeron to merge