Fix buck2 build error in xplat/thrift

[bujar]

Summary: Error

[2023-04-03T10:54:13.480-07:00] Stderr: xplat/thrift/lib/cpp/test/Base64Test.cpp:37:6: error: no previous prototype for function 'checkEncoding' [-Werror,-Wmissing-prototypes]
[2023-04-03T10:54:13.480-07:00] void checkEncoding(uint8_t* data, int len) {
[2023-04-03T10:54:13.480-07:00]      ^' in /var/sandcastle/deployment/mount/53446/flib/intern/sandcastle/core/shell/SandcastleStatusDecoderBase.php:279

This function needs to be static for it to be considered a prototype

Differential Revision: D44649127

[facebook-github-bot]

This pull request was exported from Phabricator. Differential Revision: D44649127

[facebook-github-bot]

This pull request has been merged in facebook/hhvm@a1ffc538111d9cec6978a2134eb58e0336fb28fa.