Fix skslc compilation on Windows

Change-Id: Id3762e77df0b1fe69bee1e57d9ccef907279fcb3
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/347048
Auto-Submit: Brian Osman <brianosman@google.com>
Commit-Queue: John Stiles <johnstiles@google.com>
Reviewed-by: John Stiles <johnstiles@google.com>
diff --git a/BUILD.gn b/BUILD.gn
index fc20ab0..fc9d1b5 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -568,6 +568,7 @@
       "src/core/SkSemaphore.cpp",
       "src/core/SkStream.cpp",
       "src/core/SkString.cpp",
+      "src/core/SkStringUtils.cpp",
       "src/core/SkThreadID.cpp",
       "src/core/SkUtils.cpp",
       "src/core/SkVM.cpp",