Needed by the fix for CVE-2025-15661

Cherrypicked from 
commit 606c102e52f8447de2b745dd6c5ddf418defc519
Author: Viktor Szakats <commit@vsz.me>
Date:   Thu Jan 30 21:18:23 2025 +0100

--- libssh2-1.11.1.orig/src/libssh2_priv.h
+++ libssh2-1.11.1/src/libssh2_priv.h
@@ -117,6 +117,14 @@
 #define UINT32_MAX 0xffffffffU
 #endif
 
+#ifdef _WIN64
+#define LIBSSH2_UNCONST(p)  ((void *)(libssh2_uint64_t)(const void *)(p))
+#elif defined(_MSC_VER)
+#define LIBSSH2_UNCONST(p)  ((void *)(unsigned int)(const void *)(p))
+#else
+#define LIBSSH2_UNCONST(p)  ((void *)(uintptr_t)(const void *)(p))
+#endif
+
 #if (defined(__GNUC__) || defined(__clang__)) && \
     defined(__STDC_VERSION__) && (__STDC_VERSION__ >= 199901L) && \
     !defined(LIBSSH2_NO_FMT_CHECKS)