vp9_ext_ratectrl_test: use uintptr_t for void* value

this avoids a warning about differences in size between void* and
unsigned int under msvc:
vp9_ext_ratectrl_test.cc(40,3): warning C4312: 'reinterpret_cast':
conversion from 'const unsigned int' to 'void *' of greater size

Change-Id: I5a412ec785ddcaeff2ec71bb83a6048505400293
1 file changed