Fix gdb.base/list.exp with Clang

Two subtests of gdb.base/list.exp failed when built with Clang
because the unused function "unused" was optimized out.  This
commit adds __attribute__ ((used)) to both definitions.

gdb/testsuite/ChangeLog:

	* gdb.base/list0.c (unused): Add __attribute__ ((used)).
	* gdb.base/list1.c (unused): Likewise.