ghci: fix isMinTTY.h casing for Windows targets

This commit fixes isMinTTY.h casing in isMinTTY.c that's compiled for
Windows targets. While this looks harmless given Windows filesystems
are case-insensitive by default, it does cause a compilation warning
with recent versions of clang, so we might as well fix the casing:

```
driver\ghci\isMinTTY.c:10:10: error:
     warning: non-portable path to file '"isMinTTY.h"'; specified path differs in case from file name on disk [-Wnonportable-include-path]
   |
10 | #include "isMINTTY.h"
   |          ^

 #include "isMINTTY.h"
         ^~~~~~~~~~~~
         "isMinTTY.h"
1 warning generated.
```