Fix installation failure due to an error when reading ASD files

[fukamachi]

CFFI can't be added because of a read-time error in cffi-libffi that loads cffi-grovel in the ASD file and uses its symbol.

https://github.com/cffi/cffi/blob/65a5321e13a25ad057ec1dd5670df4b174678369/cffi-libffi.asd#L44

Reading '/tmp/qlot-test/qlfile'...
⨯ [1/1] cffi  Failed to install

Unexpected error: Package CFFI-GROVEL does not exist.

                    Stream: #<SB-SYS:FD-STREAM for "file /tmp/qlot-504LBAXJ/cffi-ref-65a5321e13a25ad057ec1dd5670df4b174678369/cffi-libffi.asd" {100589AB13}>
This can be a bug of Qlot.
Report it at https://github.com/fukamachi/qlot/issues/new/choose.
Please attach the stack trace dumped to '/tmp/qlot-error-8V3J6PE9.log'.