build: set D_WIN32_WINNT=0x0601 for mingw miniupnpc

This matches configure, and what we set for libevent etc.

depends/packages/miniupnpc.mk

@@ -5,10 +5,12 @@ $(package)_file_name=$(package)-$($(package)_version).tar.gz
 $(package)_sha256_hash=888fb0976ba61518276fe1eda988589c700a3f2a69d71089260d75562afd3687
 $(package)_patches=dont_leak_info.patch respect_mingw_cflags.patch
 
+# Next time this package is updated, ensure that _WIN32_WINNT is still properly set.
+# See discussion in https://github.com/bitcoin/bitcoin/pull/25964.
 define $(package)_set_vars
 $(package)_build_opts=CC="$($(package)_cc)"
 $(package)_build_opts_darwin=LIBTOOL="$($(package)_libtool)"
-$(package)_build_opts_mingw32=-f Makefile.mingw
+$(package)_build_opts_mingw32=-f Makefile.mingw CFLAGS="$($(package)_cflags) -D_WIN32_WINNT=0x0601"
 $(package)_build_env+=CFLAGS="$($(package)_cflags) $($(package)_cppflags)" AR="$($(package)_ar)"
 endef