[BUILD] Add missing file dependency relations

[claui]

This PR fixes parallel builds.
Tested with -j1, -j2, and -j16.
Might need testing on at least one more machine.

Fixes #363.

[claui]

Need to fix make clean, which fails to clean up nested basload and x16-edit builds.

[claui]

Output of make clean && make -j2:

Before

[…]
(cd x16-edit && make clean rom)
(cd basload && make clean && make)
make[1]: Entering directory '/home/claudia/Documents/dev/x16-rom/x16-edit'
rm -f build/*
make[1]: Nothing to be done for 'rom'.
make[1]: Leaving directory '/home/claudia/Documents/dev/x16-rom/x16-edit'
cp x16-edit/build/x16edit-rom.bin build/x16/x16edit-rom.bin
make[1]: Entering directory '/home/claudia/Documents/dev/x16-rom/basload'
rm -f build/*
cp: cannot stat 'x16-edit/build/x16edit-rom.bin': No such file or directory
make: *** [Makefile:482: build/x16/x16edit-rom.bin] Error 1
[…]

After

(cd basload && make clean)
make[1]: Entering directory '/home/claudia/Documents/dev/x16-rom/basload'
rm -f build/*
make[1]: Leaving directory '/home/claudia/Documents/dev/x16-rom/basload'
(cd x16-edit && make clean)
make[1]: Entering directory '/home/claudia/Documents/dev/x16-rom/x16-edit'
rm -f build/*
make[1]: Leaving directory '/home/claudia/Documents/dev/x16-rom/x16-edit'
rm -f build/x16/../signature.bin
rm -rf build/x16
[…]
(cd x16-edit && make clean rom)
(cd basload && make clean && make)
make[1]: Entering directory '/home/claudia/Documents/dev/x16-rom/x16-edit'
rm -f build/*
make[1]: Entering directory '/home/claudia/Documents/dev/x16-rom/basload'
rm -f build/*
lzsa -r -f2 help.txt build/help.bin
make[1]: Leaving directory '/home/claudia/Documents/dev/x16-rom/basload'
make[1]: Entering directory '/home/claudia/Documents/dev/x16-rom/basload'
cl65 -o build/basload-rom.bin --cpu 65C02 -t none -C conf/basload-rom.cfg -m build/basload-rom.map -l build/basload-rom.lst -Ln build/basload-rom.sym main.asm
lzsa -r -f2 help_short.txt build/help_short.bin
cl65 --asm-args -Dtarget_mem=2 -g -o build/x16edit-rom.bin -t cx16 -C conf/x16edit-rom.cfg --mapfile build/x16edit-rom.map -Ln build/x16edit-rom.sym -l build/x16edit-rom.lst main.asm
rm -f main.o
make[1]: Leaving directory '/home/claudia/Documents/dev/x16-rom/basload'
[…]
cp x16-edit/build/x16edit-rom.bin build/x16/x16edit-rom.bin
./scripts/trace_info.py 13 x16-edit/conf/x16edit-rom.cfg x16-edit/build/x16edit-rom.lst  build/x16/x16edit-rom_D.rlst build/x16/x16edit_D_labels.h
cp basload/build/basload-rom.bin build/x16/basload-rom.bin
./scripts/trace_info.py 15 basload/conf/basload-rom.cfg basload/build/basload-rom.lst build/x16/basload-rom.rlst build/x16/basload_labels.h
[…]

Works as expected.