Add descriptor structure definition

sdk/xtdk/i686/xtstruct.h

@@ -22,6 +22,7 @@ typedef struct _CPUID_REGISTERS CPUID_REGISTERS, *PCPUID_REGISTERS;
 typedef struct _FLOATING_SAVE_AREA FLOATING_SAVE_AREA, *PFLOATING_SAVE_AREA;
 typedef struct _HARDWARE_PTE HARDWARE_PTE, *PHARDWARE_PTE;
 typedef struct _HARDWARE_PTE_PAE HARDWARE_PTE_PAE, *PHARDWARE_PTE_PAE;
+typedef struct _KDESCRIPTOR KDESCRIPTOR, *PKDESCRIPTOR;
 typedef struct _KEXCEPTION_FRAME KEXCEPTION_FRAME, *PKEXCEPTION_FRAME;
 typedef struct _KGDTENTRY KGDTENTRY, *PKGDTENTRY;
 typedef struct _KIDTENTRY KIDTENTRY, *PKIDTENTRY;