cmdutils: Fix compilation on cygwin wrt SetDllDirectory and GetModuleHandle

cmdutils: Fix compilation on cygwin wrt SetDllDirectory and GetModuleHandle

After 06ec9c4746ebc1d3b613c1847e434bbd0b4407b4 we check for these
functions in configure (which will succeed in cygwin), but cmdutils.c
only includes windows.h if _WIN32 is defined (which it isn't in cygwin).

Retain the old intent from before 06ec9c4746ebc1d3b613c1847e434bbd0b4407b4,
that these functions only would be used when _WIN32 is defined, while
only using them if configure has agreed that they do exist.

Signed-off-by: Martin Storsjö <martin@martin.st>