Eliminate 'static GC_sysinfo definition has incomplete type' Clang warning

Eliminate 'static GC_sysinfo definition has incomplete type' Clang warning

Tentative definition of variable with internal linkage has incomplete
non-array type 'struct _SYSTEM_INFO'.

The warning observed only in single-threaded shared builds by Clang.

* include/private/gc_priv.h [MSWIN32 || MSWINCE || CYGWIN32]
(WIN32_LEAN_AND_MEAN, NOSERVICE): Define macro.
* include/private/gc_priv.h [MSWIN32 || MSWINCE || CYGWIN32]: Include
windows.h (wrapped into EXTERN_C_END/BEGIN) before GC_sysinfo
declaration.
* include/private/gc_priv.h [MSWIN32 || MSWINCE || CYGWIN32]
(_SYSTEM_INFO): Do not declare.
* include/private/gc_priv.h [MSWIN32 || MSWINCE || CYGWIN32]
(GC_sysinfo): Change type from struct _SYSTEM_INFO to SYSTEM_INFO.