boost: fix wrong type for mutex in regex v5

Fixes:
 | ./boost/regex/v5/mem_block_cache.hpp:91:11: error: 'static_mutex' in namespace 'boost' does not name a type
 |    91 |    boost::static_mutex mut;

Signed-off-by: Mingli Yu <mingli.yu@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>