Improve skcms BUILD.gn file
Improve skcms BUILD.gn file

Followup to reviews.skia.org/554056

Change-Id: I0ef64c320eb1dcb150855822f84619983eae38c5
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/562298
Reviewed-by: Brian Osman <794c0b5534edf5601d88e1d41975d0262da12894@google.com>

google/skia,google/skia,google/skia,google/skia,google/skia,google/skia,google/skia,google/skia,google/skia,google/skia