build: fix cflags passing for mingw miniupnpc

Cherry-pick of a patch upstreamed to miniupnpc, see here:
https://github.com/miniupnp/miniupnp/pull/619.

depends/packages/miniupnpc.mk

@@ -3,7 +3,7 @@ $(package)_version=2.2.2
 $(package)_download_path=https://miniupnp.tuxfamily.org/files/
 $(package)_file_name=$(package)-$($(package)_version).tar.gz
 $(package)_sha256_hash=888fb0976ba61518276fe1eda988589c700a3f2a69d71089260d75562afd3687
-$(package)_patches=dont_leak_info.patch
+$(package)_patches=dont_leak_info.patch respect_mingw_cflags.patch
 
 define $(package)_set_vars
 $(package)_build_opts=CC="$($(package)_cc)"
@@ -13,7 +13,8 @@ $(package)_build_env+=CFLAGS="$($(package)_cflags) $($(package)_cppflags)" AR="$
 endef
 
 define $(package)_preprocess_cmds
-  patch -p1 < $($(package)_patch_dir)/dont_leak_info.patch
+  patch -p1 < $($(package)_patch_dir)/dont_leak_info.patch && \
+  patch -p1 < $($(package)_patch_dir)/respect_mingw_cflags.patch
 endef
 
 define $(package)_build_cmds