vulkan: replace std::aligned_alloc with posix_memalign for termux compatibility (#30828)

std::aligned_alloc is unavailable on Android bionic below API 28 (Termux targets API 24); use posix_memalign like ggml.c does.

ggml/src/ggml-vulkan/ggml-vulkan-buffers.cpp

@@ -9,7 +9,11 @@ static void * portable_aligned_malloc(size_t align, size_t size) {
 #ifdef _WIN32
     return _aligned_malloc(size, align);
 #else
-    return std::aligned_alloc(align, size);
+    void * ptr = nullptr;
+    if (posix_memalign(&ptr, align, size) != 0) {
+        return nullptr;
+    }
+    return ptr;
 #endif
 }