Fix cc65 cpu.mac multiple inclusion error

- Create common/inc/cpu.inc with include guard for cpu macros
- Update apple2/apple2-6502/bus/sp_init.s to use guarded include
- Prevents 'Symbol already defined' errors when cpu.mac is included multiple times

This fixes the issue where ca65 was redefining CPU_ISET_* symbols
during compilation of assembly files.

Author: dillera

apple2/apple2-6502/bus/sp_init.s

@@ -10,7 +10,7 @@
 
         .include        "macros.inc"
         .include        "zp.inc"
-        .macpack        cpu
+        .include        "cpu.inc"
 
 ; Find the SmartPort device that has a FujiNet NETWORK adapter on it.
 ; Really we should search for the FUJI device on it, but historically that was

common/inc/cpu.inc

@@ -0,0 +1,7 @@
+; Guard to prevent multiple inclusion of cpu macros
+.ifndef __CPU_INC_INCLUDED__
+.define __CPU_INC_INCLUDED__
+
+.macpack cpu
+
+.endif