lib/uuid: remove unneeded Windows UUID workaround

lib/uuid: remove unneeded Windows UUID workaround

Some .c files in lib/uuid/ contain the following:

	#ifdef _WIN32
	#define _WIN32_WINNT 0x0500
	#include <windows.h>
	#define UUID MYUUID
	#endif

This seems to have been intended to allow the use of a local "UUID" type
without colliding with "UUID" in the Windows API.  However, this is
unnecessary because there's no local "UUID" type -- there's only uuid_t.

None of these .c files need the include of windows.h, either.

Finally, the unconditional definition of _WIN32_WINNT causes a compiler
warning when the user defines _WIN32_WINNT themself.

Since this code is unnecessary and is causing problems, just remove it.

Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>