libstdc++: Include <mutex> in syncbuf.cc [PR122698]

libstdc++: Include <mutex> in syncbuf.cc [PR122698]

For most configurations bits/std_mutex.h would already be included by
<syncstream>, but not if configured with _GLIBCXX_USE_CXX11_ABI=0 as the
default, because syncbuf is disabled in that case.

libstdc++-v3/ChangeLog:

	PR libstdc++/122698
	* src/c++20/syncbuf.cc (__syncbuf_get_mutex): Include <mutex>.
	Fix indentation of function body.