i18n: csmapper: hook up the top-level .src files to the build

The top-level charset.pivot.src may be empty, but the top-level
mapper.dir.src is not -- it contains some definitions for supplementary
UCS planes. The SMP/SSP definitions aren't actually used at the moment,
but the inclusion of the SIP definitions fixes conversions involving
JISX0213-1/JISX0213-2 to be possible.

Sponsored by: Klara, Inc.

(cherry picked from commit dc6df546be29a468ae21b1b7604e85b703ad6779)