memmod: apply explicit build tags to _32 and _64 files

Since _32 and _64 aren't valid goarchs, they don't match _GOOS_GOARCH,
and so the existing tags wind up not being restricted to windows-only.
This fixes the problem by adding windows to the tags explicitly. We
could also fix it by calling the files _32_windows or _64_windows, but
that changes the convention with the other single-arch files.

Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>

tun/wintun/memmod/memmod_windows_32.go

@@ -1,4 +1,4 @@
-// +build 386 arm
+// +build windows,386 windows,arm
 
 /* SPDX-License-Identifier: MIT
  *

tun/wintun/memmod/memmod_windows_64.go

@@ -1,4 +1,4 @@
-// +build amd64 arm64
+// +build windows,amd64 windows,arm64
 
 /* SPDX-License-Identifier: MIT
  *

tun/wintun/memmod/syscall_windows_32.go

@@ -1,4 +1,4 @@
-// +build 386 arm
+// +build windows,386 windows,arm
 
 /* SPDX-License-Identifier: MIT
  *

tun/wintun/memmod/syscall_windows_64.go

@@ -1,4 +1,4 @@
-// +build amd64 arm64
+// +build windows,amd64 windows,arm64
 
 /* SPDX-License-Identifier: MIT
  *