BLD: Fix msvc error C2036: 'void *' : unknown size

The `scipy.ndimage._ni_label` extension from scipy 0.13.0b1 fails to build with Visual Studio compilers:
```
<snip>
building 'scipy.ndimage._ni_label' extension
compiling C sources
C:\Program Files (x86)\Microsoft Visual Studio 9.0\VC\BIN\cl.exe /c /nologo /Ox /MD /W3 /GS- /DNDEBUG  -Iscipy\ndimage\s
rc -IX:\Python27\lib\site-packages\numpy\core\include -IX:\Python27\lib\site-packages\numpy\core\include -IX:\Python27\i
nclude -IX:\Python27\PC /Tcscipy\ndimage\src\_ni_label.c /Fobuild\temp.win32-2.7\Release\scipy\ndimage\src\_ni_label.obj

_ni_label.c
x:\python27\lib\site-packages\numpy\core\include\numpy\npy_deprecated_api.h(8) : Warning Msg: Using deprecated NumPy API
, disable it by #defining NPY_NO_DEPRECATED_API NPY_1_7_API_VERSION
scipy\ndimage\src\_ni_label.c(2168) : warning C4018: '<' : signed/unsigned mismatch
scipy\ndimage\src\_ni_label.c(2178) : error C2036: 'void *' : unknown size
scipy\ndimage\src\_ni_label.c(2234) : warning C4018: '<' : signed/unsigned mismatch
scipy\ndimage\src\_ni_label.c(2244) : error C2036: 'void *' : unknown size
scipy\ndimage\src\_ni_label.c(2300) : warning C4018: '<' : signed/unsigned mismatch
scipy\ndimage\src\_ni_label.c(2310) : error C2036: 'void *' : unknown size
scipy\ndimage\src\_ni_label.c(2366) : warning C4018: '<' : signed/unsigned mismatch
scipy\ndimage\src\_ni_label.c(2376) : error C2036: 'void *' : unknown size
scipy\ndimage\src\_ni_label.c(2432) : warning C4018: '<' : signed/unsigned mismatch
scipy\ndimage\src\_ni_label.c(2442) : error C2036: 'void *' : unknown size
scipy\ndimage\src\_ni_label.c(2498) : warning C4018: '<' : signed/unsigned mismatch
scipy\ndimage\src\_ni_label.c(2508) : error C2036: 'void *' : unknown size
scipy\ndimage\src\_ni_label.c(2564) : warning C4018: '<' : signed/unsigned mismatch
scipy\ndimage\src\_ni_label.c(2574) : error C2036: 'void *' : unknown size
scipy\ndimage\src\_ni_label.c(2630) : warning C4018: '<' : signed/unsigned mismatch
scipy\ndimage\src\_ni_label.c(2640) : error C2036: 'void *' : unknown size
scipy\ndimage\src\_ni_label.c(2696) : warning C4018: '<' : signed/unsigned mismatch
scipy\ndimage\src\_ni_label.c(2706) : error C2036: 'void *' : unknown size
scipy\ndimage\src\_ni_label.c(2762) : warning C4018: '<' : signed/unsigned mismatch
scipy\ndimage\src\_ni_label.c(2772) : error C2036: 'void *' : unknown size
<snip>
```
The proposed fix uses `char *` instead of `void *` and passes all relevant tests.