Working PIT irqs, fix GDT bugs

kernel/platform/i386_pc/sys/make.src

@@ -6,7 +6,8 @@ c_files += $(dir_sys)/cpu.c \
            $(dir_sys)/ioport.c \
            $(dir_sys)/isr1.c \
            $(dir_sys)/pic.c \
-           $(dir_sys)/mm.c
+           $(dir_sys)/mm.c \
+           $(dir_sys)/pit.c
 
 S_files += $(dir_sys)/gdt_flush.S \
            $(dir_sys)/tss_flush.S \
@@ -25,4 +26,5 @@ o_files += $(dir_sys)/cpu.o \
            $(dir_sys)/isr1.o \
            $(dir_sys)/halt.o \
            $(dir_sys)/pic.o \
-           $(dir_sys)/mm.o
+           $(dir_sys)/mm.o \
+           $(dir_sys)/pit.o