sql: move sql_stmt_finalize() to execute.h

We are going to introduce prepared statement cache. On statement's
deallocation we should release all resources which is done by
sql_stmt_finalize(). Now it is declared in sql/sqlInt.h header, which
accumulates almost all SQL related functions. To avoid including such a
huge header to use single function, let's move its signature to
box/execute.h

Needed for #2592