internal: don't use weak symbols for Win32 platform

internal: don't use weak symbols for Win32 platform

The Win32 platform will fail to link if you use weak symbols
because it is incompatible with exporting symbols in a DLL:

Cannot export virRandomGenerateWWN: symbol wrong type (2 vs 3)

We only need weak symbols for our test suite to do LD_PRELOAD
and this doesn't work on Win32, so we can just drop the hack
for Win32

Signed-off-by: Daniel P. Berrange <berrange@redhat.com>