Move conio.h from win32.h to mingw.c

We need conio.h to avoid a warning about the implicit declaration
of _getch().  Including it in mingw.c is sufficient.

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;

compat/win32.h

@@ -1,6 +1,5 @@
 /* common Win32 functions for MinGW and Cygwin */
 #include <windows.h>
-#include <conio.h>
 
 static inline int file_attr_to_st_mode (DWORD attr)
 {