Use rvalue reference in thread_pool::post_task

Use rvalue reference in thread_pool::post_task

Tankut's recent patches made me realize that thread_pool::post_task
should have used an rvalue reference for its parameter.  This patch
makes this change.

gdbsupport/ChangeLog
2021-04-30  Tom Tromey  <tromey@adacore.com>

	* thread-pool.cc (thread_pool::post_task): Update.
	* thread-pool.h (class thread_pool) <post_task>: Take rvalue
	reference to function.