swscale: Do not expand a macro with 'defined' in it

Fixes:

    libswscale/utils.c:1632:5: warning: macro expansion producing 'defined' has undefined behavior [-Wexpansion-to-defined]
    #if USE_MMAP
        ^
    libswscale/utils.c:1577:49: note: expanded from macro 'USE_MMAP'
    #define USE_MMAP (HAVE_MMAP && HAVE_MPROTECT && defined MAP_ANONYMOUS)
                                                    ^
Signed-off-by: Derek Buitenhuis <derek.buitenhuis@gmail.com>
Signed-off-by: Luca Barbato <lu_zero@gentoo.org>