Workaround build bug with GCC 6.2.1

Workaround build bug with GCC 6.2.1

Building GDB with GCC 6.2.1 gives multiple errors like

gdb/dtrace-probe.c: In member function ‘void dtrace_probe::build_arg_exprs(gdbarch*)’:
gdb/dtrace-probe.c:627:8: error: types may not be defined in a for-range-declaration [-Werror]
    for (struct dtrace_probe_arg &arg : m_args

Fix it by removing the 'struct' keyword.

A similar Bug was already fixed for GCC 6.3.1
https://sourceware.org/ml/gdb-patches/2017-10/msg00442.html

gdb/ChangeLog:

	* dtrace-probe.c (dtrace_probe::build_arg_exprs)
	(dtrace_probe::is_enabled, dtrace_probe::enable)
	(dtrace_probe::disable): Remove keyword 'struct' at for-range
	variable
	* probe.c (gen_ui_out_table_header_info)
	(print_ui_out_not_applicables):  Remove keyword 'struct' at
	for-range variable