Make qemu compile with Linux 2.6.2 headers

I couldn't compile today's CVS without the following.

Juliusz Chroboczek

Index: target-i386/helper2.c
===================================================================
RCS file: /cvsroot/qemu/qemu/target-i386/helper2.c,v retrieving revision 1.15

diff -u -r1.15 helper2.c
@@ -94,7 +94,7 @@
 #ifdef USE_CODE_COPY
     /* testing code for code copy case */
     {
-        struct modify_ldt_ldt_s ldt;
+        struct user_desc ldt;
 
         ldt.entry_number = 1;
         ldt.base_addr = (unsigned long)env;