Added spec entry for _itoa.

diff --git a/dlls/ntdll/ntdll.spec b/dlls/ntdll/ntdll.spec
index c877146..dac2506 100644
--- a/dlls/ntdll/ntdll.spec
+++ b/dlls/ntdll/ntdll.spec
@@ -870,7 +870,7 @@
 @ register _chkstk() NTDLL_chkstk
 @ stub _fltused
 @ cdecl _ftol(double) CRTDLL__ftol
-@ stub _itoa
+@ cdecl _itoa(long ptr long) CRTDLL__itoa
 @ stub _ltoa
 @ stub _memccpy
 @ cdecl _memicmp(str str long) CRTDLL__memicmp