[fuzzer] Fix building on case sensitive mingw platforms

Include windows.h with an all lowercase filename; Windows SDK headers
aren't self consistent so they can't be used in an entirely
case sensitive setting, and mingw headers use all lowercase names
for such headers.

This fixes building after 881faf41909b47376595e8d7bb9c9a109182d20b.

GitOrigin-RevId: ab737d5367cdea30df71ef66f87fe2c6d88e7246
diff --git a/FuzzerExtraCountersWindows.cpp b/FuzzerExtraCountersWindows.cpp
index 1910184..102f5fe 100644
--- a/FuzzerExtraCountersWindows.cpp
+++ b/FuzzerExtraCountersWindows.cpp
@@ -12,7 +12,7 @@
 #include <cstdint>
 
 #if LIBFUZZER_WINDOWS
-#include <Windows.h>
+#include <windows.h>
 
 namespace fuzzer {