Move includes out of src

This mirrors gb-starter-kit, where it was important because submodules
can pull in `.asm` files that we don't want to assemble.

Makefile

@@ -23,7 +23,7 @@ RGBGFX  := ${RGBDS}rgbgfx
 ROM = bin/${ROMNAME}.${ROMEXT}
 
 # Argument constants
-INCDIRS  = src/ src/include/
+INCDIRS  = src/ include/
 WARNINGS = all extra
 ASFLAGS  = -p ${PADVALUE} $(addprefix -I,${INCDIRS}) $(addprefix -W,${WARNINGS})
 LDFLAGS  = -p ${PADVALUE}