i386: avoid a write only variable

Compiling with GCC 4.6.0 20100925 produced warnings:
/src/qemu/target-i386/op_helper.c: In function 'switch_tss':
/src/qemu/target-i386/op_helper.c:283:53: error: variable 'new_trap' set but not used [-Werror=unused-but-set-variable]

Fix by adding a dummy cast so that the variable is not unused. Add also
pointer to docs.
Signed-off-by: NBlue Swirl <blauwirbel@gmail.com>

target-i386/op_helper.c

@@ -349,6 +349,10 @@ static void switch_tss(int tss_selector,
         new_segs[R_GS] = 0;
         new_trap = 0;
     }
+    /* XXX: avoid a compiler warning, see
+     http://support.amd.com/us/Processor_TechDocs/24593.pdf
+     chapters 12.2.5 and 13.2.4 on how to implement TSS Trap bit */
+    (void)new_trap;
 
     /* NOTE: we must avoid memory exceptions during the task switch,
        so we make dummy accesses before */