vulkan: don't load core extensions

Fix SIGABRT on Android emulator:
```
terminating due to uncaught exception of type vk::ExtensionNotPresentError: vk::PhysicalDevice::createDevice: ErrorExtensionNotPresent
```

Author: tguillem

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

@@ -4794,7 +4794,9 @@ static vk_device ggml_vk_get_device(size_t idx) {
             throw std::runtime_error("Unsupported device");
         }
 
-        device_extensions.push_back("VK_KHR_16bit_storage");
+        if (fp16_storage) {
+            device_extensions.push_back("VK_KHR_16bit_storage");
+        }
 
 #ifdef GGML_VULKAN_VALIDATE
         device_extensions.push_back("VK_KHR_shader_non_semantic_info");