added initial GLSL support to skslc

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2185393003

Review-Url: https://codereview.chromium.org/2185393003
diff --git a/src/sksl/sksl.flex b/src/sksl/sksl.flex
index cbc938a..67b48e9 100644
--- a/src/sksl/sksl.flex
+++ b/src/sksl/sksl.flex
@@ -68,6 +68,10 @@
 
 highp { return SkSL::Token::HIGHP; }
 
+flat { return SkSL::Token::FLAT; }
+
+noperspective { return SkSL::Token::NOPERSPECTIVE; }
+
 struct { return SkSL::Token::STRUCT; }
 
 layout { return SkSL::Token::LAYOUT; }