fixed cpp build error with adapters/libhv.h

[mtdxc]

small fixed when include dapters/libhv.h in cpp files in vs2017:

error C2664: “void redisAsyncHandleTimeout(redisAsyncContext *)”: 无法将参数 1 从“void *”转换为“redisAsyncContext *”

[bjosv]

LGTM, but interesting with the difference..

Tested it in our CI as well (which uses VS2022) where in C /Wall works fine, but building
our example-libhv example using C++ it gives:

"D:\a\hiredis\hiredis\examples\example-libhv.vcxproj" (default target) (5) ->
(ClCompile target) -> 
  D:\a\hiredis\hiredis\adapters/libhv.h(59,48): error C2664: 'void redisAsyncHandleTimeout(redisAsyncContext *)': cannot convert argument 1 from 'void *' to 'redisAsyncContext *' [D:\a\hiredis\hiredis\examples\example-libhv.vcxproj]
  D:\a\hiredis\hiredis\adapters/libhv.h(97,40): error C2440: '=': cannot convert from 'void *' to 'redisLibhvEvents *' [D:\a\hiredis\hiredis\examples\example-libhv.vcxproj]```

[michael-grunder]

Tested it in our CI as well (which uses VS2022) where in C /Wall works fine, but building
our example-libhv example using C++ it gives:

Yeah, I always forget about C++'s explicit typecast requirement.