fix(gatsby-plugin-mdx): add missing change-case dependency (#15638)

Author: DSchau

packages/gatsby-plugin-mdx/package.json

@@ -22,6 +22,7 @@
     "@babel/plugin-proposal-object-rest-spread": "^7.4.3",
     "@babel/preset-env": "^7.4.3",
     "@babel/preset-react": "^7.0.0",
+    "change-case": "^3.1.0",
     "core-js": "2",
     "dataloader": "^1.4.0",
     "debug": "^4.0.1",

yarn.lock

@@ -5784,7 +5784,7 @@ change-case@^3.0.1:
     upper-case "^1.1.1"
     upper-case-first "^1.1.0"
 
-change-case@^3.0.2:
+change-case@^3.0.2, change-case@^3.1.0:
   version "3.1.0"
   resolved "https://registry.yarnpkg.com/change-case/-/change-case-3.1.0.tgz#0e611b7edc9952df2e8513b27b42de72647dd17e"
   integrity sha512-2AZp7uJZbYEzRPsFoa+ijKdvp9zsrnnt6+yFokfwEpeJm0xuJDVoxiRCAaTzyJND8GJkofo2IcKWaUZ/OECVzw==