[binutils-gdb/gdb-8.0-branch] Avoid compiler warning in MinGW build

[sergiodj+buildbot@sergiodj.net]

*** TEST RESULTS FOR COMMIT db9a60402429d9bed9b531f7d1584860babe2c22 ***

Author: Eli Zaretskii <eliz@gnu.org>
Branch: gdb-8.0-branch
Commit: db9a60402429d9bed9b531f7d1584860babe2c22

Avoid compiler warning in MinGW build

gdb:

2017-05-13  Eli Zaretskii  <eliz@gnu.org>

	* tui/tui.c (tui_enable): Cast "unknown" to 'char *' to avoid a
	C++ compiler warning.

(cherry picked from commit adf3dde510088ef8dc46d04df05baf36adb0ed1e)