Add bazel to the list of acceptable mergeable tags

.github/mergeable.yml

@@ -11,7 +11,7 @@ mergeable:
                 regex: 'release notes: yes'
                 message: 'Please include release notes: yes'
             - must_include:
-                regex: '^(c#|c\+\+|cleanup|conformance tests|integration|java|javascript|go|objective-c|php|python|ruby)'
+                regex: '^(c#|c\+\+|cleanup|conformance tests|integration|java|javascript|go|objective-c|php|python|ruby|bazel)'
                 message: 'Please include at least a language label (e.g., c++, java, python). Or apply one of the following labels: cleanup, conformance tests, integration.'
           - must_include:
               regex: 'release notes: no'