src.libnames.mk: fix LIBPFCTL definition

Following the convention used in the rest of this file, ${LIBPFCTLDIR}
should refer to the directory, and ${LIBPFCTL} to the library itself.

Instead, both values were assigned to ${LIBPFCTL}, and ${LIBPFCTLDIR} was
not set at all.

This appears to be a simple typo and not a deliberate choice, so fix it
by assigning the directory name to ${LIBPFCTLDIR} instead.