MinGW: fix warning about implicit declaration of _getch()

conio.h provides the declaration.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Steffen Prohaska <prohaska@zib.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: dscho

compat/mingw.c

@@ -1,5 +1,6 @@
 #include "../git-compat-util.h"
 #include "win32.h"
+#include <conio.h>
 #include "../strbuf.h"
 
 unsigned int _CRT_fmode = _O_BINARY;