dsp: add missing alu.vhdl in makefile

dsp/makefile

@@ -11,7 +11,7 @@ boot_rom.gen.vhdl: main.o ../uart/uart.o
 
 sim_sources = dsp_test.vhdl
 sources = boot_rom.gen.vhdl dsp.vhdl      \
-	  ../cpu/cpu.vhdl ../cpu/reg.vhdl \
+	  ../cpu/cpu.vhdl ../cpu/reg.vhdl ../cpu/alu.vhdl \
 	  $(wildcard ../wave/*.vhdl)      \
 	  $(wildcard ../sysbus/*.vhdl)    \
 	  ../uart/uart.vhdl               \