Remove struct keyword in range-based for-loop

Fix this with gcc 6.3.0, and make the loop variable const while at it:

/home/simark/src/binutils-gdb/gdb/record-full.c: In member function 'virtual int record_full_target::insert_breakpoint(gdbarch*, bp_target_info*)':
/home/simark/src/binutils-gdb/gdb/record-full.c:1789:8: error: types may not be defined in a for-range-declaration [-Werror]
   for (struct record_full_breakpoint &bp : record_full_breakpoints)

gdb/ChangeLog:

	* record-full.c (record_full_target::insert_breakpoint): Remove
	"struct" keyword, add const.