vendor: update buildkit to v0.8

Signed-off-by: Tonis Tiigi <tonistiigi@gmail.com>

vendor/github.com/mitchellh/mapstructure/CHANGELOG.md

@@ -1,3 +1,35 @@
+## 1.3.1
+
+* Squash should only squash embedded structs. [GH-194]
+
+## 1.3.0
+
+* Added `",omitempty"` support. This will ignore zero values in the source
+  structure when encoding. [GH-145]
+
+## 1.2.3
+
+* Fix duplicate entries in Keys list with pointer values. [GH-185]
+
+## 1.2.2
+
+* Do not add unsettable (unexported) values to the unused metadata key
+  or "remain" value. [GH-150]
+
+## 1.2.1
+
+* Go modules checksum mismatch fix
+
+## 1.2.0
+
+* Added support to capture unused values in a field using the `",remain"` value
+  in the mapstructure tag. There is an example to showcase usage.
+* Added `DecoderConfig` option to always squash embedded structs
+* `json.Number` can decode into `uint` types
+* Empty slices are preserved and not replaced with nil slices
+* Fix panic that can occur in when decoding a map into a nil slice of structs
+* Improved package documentation for godoc
+
 ## 1.1.2
 
 * Fix error when decode hook decodes interface implementation into interface