devel/fb303: fix build with GCC-based architectures

/usr/local/include/thrift/concurrency/ThreadManager.h:61: error: ISO C++ forbids declaration of 'function' with no type
/usr/local/include/thrift/concurrency/ThreadManager.h:61: error: typedef name may not be a nested-name-specifier
/usr/local/include/thrift/concurrency/ThreadManager.h:61: error: expected ';' before '<' token
/usr/local/include/thrift/concurrency/ThreadManager.h:191: error: 'ExpireCallback' has not been declared

Approved by: mentors (implicit approval)