libroot: Reinitialize global heap lock after fork.

The per-heap locks are reinitialized already by the routine which
invokes this one, but this global lock was previously missed.

May fix a deadlock reported by trungnt2910 to happen in the .NET port.

src/system/libroot/posix/malloc_hoard2/arch-specific.cpp

@@ -79,6 +79,8 @@ __init_after_fork(void)
 			sHeapBase);
 		exit(1);
 	}
+
+	hoardLockInit(sHeapLock, "heap");
 }