human68k: actually tell the build system where the rtl directory is

rtl/Makefile.fpc

@@ -56,6 +56,7 @@ dirs_zxspectrum=zxspectrum
 dirs_msxdos=msxdos
 dirs_sinclairql=sinclairql
 dirs_wasi=wasi
+dirs_human68k=human68k
 
 [install]
 fpcpackage=y