Avoid MinGW compilation warning in readline/input.c

This change was already accepted upstream in Readline.

readline/ChangeLog.gdb:

2017-05-19  Eli Zaretskii  <eliz@gnu.org>

	* input.c [_WIN32]: Include <conio.h> to avoid compiler warning on
	MinGW.

readline/ChangeLog.gdb

@@ -1,3 +1,8 @@
+2017-05-19  Eli Zaretskii  <eliz@gnu.org>
+
+	* input.c [_WIN32]: Include <conio.h> to avoid compiler warning on
+	MinGW.
+
 2016-09-17  Eli Zaretskii  <eliz@gnu.org>
 
 	* util.c: Include rlshell.h.

readline/input.c

@@ -91,6 +91,7 @@ static int rl_gather_tyi PARAMS((void));
 /* 'isatty' in the Windows runtime returns non-zero for every
    character device, including the null device.  Repair that.  */
 #include <io.h>
+#include <conio.h>
 #define WIN32_LEAN_AND_MEAN 1
 #include <windows.h>