Avoid compilation warnings on MinGW.

gdb/ChangeLog:
2019-03-02  Eli Zaretskii  <eliz@gnu.org>

	* xml-syscall.c (xml_list_syscalls_by_group): Drop 'struct' from
	for-loop range, to avoid compiler warnings.

	* tui/tui.c (tui_enable) [__MINGW32__]: Don't declare 'cap', to
	avoid compiler warnings about unused variables.