Avoid having LockRecursionException in both mscorlib and System.Core for the MOBILE profile. Fix bug #5311

mcs/class/System.Core/System.Threading/LockRecursionException.cs

@@ -24,7 +24,7 @@
  * WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
  */
 
-#if NET_4_0
+#if NET_4_0 || MOBILE
 
 using System;
 using System.Runtime.CompilerServices;