isolinux: only add speech synth install entry if missing

live-build commit 24aab15d2a133075992ce3a216b1c3476dc7bd4c has added
generation of speech synth entries. this ensures that with the next
release of live-build, the kali one is not added as a duplicate.

note that live-build commit a25b77e0998ac455b48c8c523a08bd282229fb49
removes "video=vesa:ywrap,mtrr" from the entries it generates, so lack of
that is deliberate should you later compare the differences and wonder
about it.

kali-config/common/hooks/live/accessibility-menu.binary

@@ -4,6 +4,7 @@ if [ ! -d isolinux ]; then
     cd binary
 fi
 
+if ! grep "speakup.synth=soft" isolinux/install.cfg >/dev/null; then
 cat >>isolinux/install.cfg <<END
 
 label installspk
@@ -12,4 +13,4 @@ label installspk
 	initrd /install/gtk/initrd.gz
 	append video=vesa:ywrap,mtrr vga=788 speakup.synth=soft --- quiet
 END
-
+fi