[gdb/testsuite] Add back missing debug for index-cache.exp

[gdb/testsuite] Add back missing debug for index-cache.exp

The proc prepare_for_testing has "debug" as default argument for the options
parameter.

In the commit c596f180a1 "[gdb/testsuite] Compile index-cache.c with
-Wl,--build-id", by setting the options argument we've effectively dropped
"debug".  This causes index-cache.exp to not contain any debug info anymore on
most systems (though not on openSUSE), which causes index-cache.exp FAILs.

Fix this by adding back the missing "debug" option.

Tested on x86_64-linux.

gdb/testsuite/ChangeLog:

2019-06-26  Tom de Vries  <tdevries@suse.de>

	* gdb.base/index-cache.exp: Add back missing debug option.