eal/windows: use lowercase filenames for system headers

eal/windows: use lowercase filenames for system headers

Mixed case in Windows header names causes errors when cross-compiling
from Linux with case-sensitive filesystem using MinGW, because MinGW
distribution provides all platform SDK headers in lowercase. The change
does not affect Windows native builds on case-insensitive filesystems
(NTFS default).

Reported-by: Narcisa Vasile <navasile@microsoft.com>
Reported-by: William Tu <u9012063@gmail.com>
Signed-off-by: Dmitry Kozlyuk <dmitry.kozliuk@gmail.com>
Tested-by: William Tu <u9012063@gmail.com>
Tested-by: Pallavi Kadam <pallavi.kadam@intel.com>
Acked-by: Pallavi Kadam <pallavi.kadam@intel.com>