msvcr100: Add ExternalContextBase::Unblock implementation.

dlls/msvcrt/concurrency.c

@@ -915,7 +915,11 @@ unsigned int __thiscall ExternalContextBase_GetScheduleGroupId(const ExternalCon
 DEFINE_THISCALL_WRAPPER(ExternalContextBase_Unblock, 4)
 void __thiscall ExternalContextBase_Unblock(ExternalContextBase *this)
 {
-    FIXME("(%p)->() stub\n", this);
+    TRACE("(%p)->()\n", this);
+
+    /* TODO: throw context_unblock_unbalanced if this->blocked goes below -1 */
+    if (!InterlockedDecrement(&this->blocked))
+        RtlWakeAddressSingle(&this->blocked);
 }
 
 DEFINE_THISCALL_WRAPPER(ExternalContextBase_IsSynchronouslyBlocked, 4)