add dlerror stubs

src/csharp/Grpc.Core/Internal/UnmanagedLibrary.cs

@@ -183,6 +183,9 @@ namespace Grpc.Core.Internal
             [DllImport("libdl.so")]
             internal static extern IntPtr dlopen(string filename, int flags);
 
+            [DllImport("libdl.so")]
+            internal static extern IntPtr dlerror();
+
             [DllImport("libdl.so")]
             internal static extern IntPtr dlsym(IntPtr handle, string symbol);
         }
@@ -192,6 +195,9 @@ namespace Grpc.Core.Internal
             [DllImport("libSystem.dylib")]
             internal static extern IntPtr dlopen(string filename, int flags);
 
+            [DllImport("libSystem.dylib")]
+            internal static extern IntPtr dlerror();
+
             [DllImport("libSystem.dylib")]
             internal static extern IntPtr dlsym(IntPtr handle, string symbol);
         }
@@ -208,6 +214,9 @@ namespace Grpc.Core.Internal
             [DllImport("__Internal")]
             internal static extern IntPtr dlopen(string filename, int flags);
 
+            [DllImport("__Internal")]
+            internal static extern IntPtr dlerror();
+
             [DllImport("__Internal")]
             internal static extern IntPtr dlsym(IntPtr handle, string symbol);
         }
@@ -222,6 +231,9 @@ namespace Grpc.Core.Internal
             [DllImport("libcoreclr.so")]
             internal static extern IntPtr dlopen(string filename, int flags);
 
+            [DllImport("libcoreclr.so")]
+            internal static extern IntPtr dlerror();
+
             [DllImport("libcoreclr.so")]
             internal static extern IntPtr dlsym(IntPtr handle, string symbol);
         }