[Cython] [cython] BUG: Avoid exporting symbols in MemoryView utility code. (#197)

[Stefan Behnel]

Hi,

this change revealed that the generated utility code functions are written
into the C code a bit too unconditionally. The numpy_memoryview test shows
lots of C compiler warnings about unused dtype conversion functions now:

https://sage.math.washington.edu:8091/hudson/job/cython-devel-tests/1165/warnings15Result/package.45/file.1478077899/

Stefan