Use lowercase for Windows header for supporting cross-compilation

Linux mingw uses lowercase headers, so cross-building on Linux for Windows
target must use the correct form. Windows is case-insensitive, so it won't
affect host-native builds.

Change-Id: Ic2140412bb367bbce83b5e2546d70e0936928c5c
Reviewed-by: André Hartmann <aha_1980@gmx.de>
Reviewed-by: David Schulz <david.schulz@qt.io>

src/plugins/compilationdatabaseprojectmanager/compilationdatabaseproject.cpp

@@ -54,7 +54,7 @@
 #include <QFileDialog>
 
 #ifdef Q_OS_WIN
-#include <Windows.h>
+#include <windows.h>
 #endif
 
 using namespace ProjectExplorer;