dependabot: Don't update wincode / solana SDK (#114)

#### Problem

The SDK has been updated to wincode v0.6, which is incompatible with
v0.5. If we update the SPL deps to use the newest SDK or wincode
versions, then they won't be able to use Agave v4.2 when those are
released, since they're still on wincode v0.5, and the bump to wincode
v0.6 will only happen for Agave v4.3.

#### Summary of changes

Ignore solana-* and wincode deps for dependabot for now.

Author: joncinque

.github/dependabot.yml

@@ -12,6 +12,11 @@ updates:
     time: "08:00"
     timezone: UTC
   open-pull-requests-limit: 6
+  ignore:
+    # Ignore Solana dependencies and wincode for now, to avoid introducing
+    # incompatibilities in libraries used by Agave
+    - dependency-name: "solana-*"
+    - dependency-name: "wincode"
 - package-ecosystem: npm
   directories:
     - "/"