gitignore: Add gcov files

.gitignore

@@ -38,3 +38,8 @@ tools/unit-test-app/build
 # AWS IoT Examples require device-specific certs/keys
 examples/protocols/aws_iot/*/main/certs/*.pem.*
 
+# gcov coverage reports
+*.gcda
+*.gcno
+coverage.info
+coverage_report/