Reduce visibility of sksl:txts

It's only referenced here by deps, so it can be private.

Change-Id: I30c381d8736f40ab339a29e69dca1e6fea801718
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/586501
Reviewed-by: Ben Wagner <bungeman@google.com>
1 file changed