installer: use relative path to icon file on Mac OSX

Author: robhagemans

packaging/osx/installer.spec

@@ -30,4 +30,4 @@ coll = COLLECT(exe,
            name='pcbasic')
 app = BUNDLE(coll,
          name='PC-BASIC.app',
-         icon='/Users/rob/pc-basic/packaging/osx/pcbasic.icns')
+         icon='pcbasic.icns')