cgraph aagerror: squash -Wswitch-default warning

cgraph aagerror: squash -Wswitch-default warning

Squashes:

  ../../lib/cgraph/scan.l: In function ‘aagerror’:
  ../../lib/cgraph/scan.l:232:14: warning: switch missing default case
    [-Wswitch-default]
    232 |         else switch (YYSTATE) {
        |              ^~~~~~

This switch is adding more information to the error message based on which
lexing state we are in, of which four are defined: `INITIAL` (the built-in start
state), `comment`, `hstring`, and `qstring`. In the `INITIAL` state there is no
extra information to add.