shells/ksh93: Attempt to fix r508705 and unbreak index

It fails basic sanity checks:

make: "/usr/ports/shells/ksh93/Makefile" line 16: Malformed conditional (${USE_GCC} == "9")
make: Fatal errors encountered -- cannot continue
make: stopped in /usr/ports/shells/ksh93

PR: 239644
Pointy hat: cy