header self-sufficiency and idempotency

Change-Id: I4ec74a358e11d85c6eaf1658efd6f1391ee5e387
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/223930
Commit-Queue: Hal Canary <halcanary@google.com>
Commit-Queue: Ben Wagner <bungeman@google.com>
Auto-Submit: Hal Canary <halcanary@google.com>
Reviewed-by: Ben Wagner <bungeman@google.com>
diff --git a/src/utils/SkCallableTraits.h b/src/utils/SkCallableTraits.h
index 7df0124..003db21 100644
--- a/src/utils/SkCallableTraits.h
+++ b/src/utils/SkCallableTraits.h
@@ -9,6 +9,7 @@
 #define SkCallableTraits_DEFINED
 
 #include <type_traits>
+#include <tuple>
 
 template <typename R, typename... Args> struct sk_base_callable_traits {
     using return_type = R;