Add ATTRIBUTE_UNUSED_RESULT to scoped_mmap

This applies ATTRIBUTE_UNUSED_RESULT to scoped_mmap::release and fixes
a couple of spots to comply.

gdb/ChangeLog
2019-03-05  Tom Tromey  <tromey@adacore.com>

	* nat/linux-btrace.c (linux_enable_bts, linux_enable_pt): Update.
	* common/scoped_mmap.h (class scoped_mmap) <release>: Add
	ATTRIBUTE_UNUSED_RESULT.