feat(pic): add pic keyboard function to isr funcs

k/isr.S

@@ -25,6 +25,7 @@ iret
     pushl   $INT_CODE;                     \
     jmp     isr;                           \
 
+// intel-defined
 ISR_NO_ERRCODE(int_de, 0)
 ISR_NO_ERRCODE(int_db, 1)
 ISR_NO_ERRCODE(int_nmi, 2)
@@ -44,3 +45,6 @@ ISR_ERRCODE(int_ac, 17)
 ISR_NO_ERRCODE(int_mc, 18)
 ISR_NO_ERRCODE(int_xm, 19)
 ISR_NO_ERRCODE(int_ve, 20)
+
+// user-defined
+ISR_NO_ERRCODE(pic_keyboard, 33)

k/isr.h

@@ -23,4 +23,7 @@ void int_mc(void);      // Machine Check
 void int_xm(void);      // SIMD Floating-Point Exception
 void int_ve(void);      // Virtualization Exception
 
+// ISR for PIC interrupts.
+void pic_keyboard(void);
+
 #endif  /* !ISR_H */