Fix compile on older systems that don't have fallocate() on Linux

Fix compile on older systems that don't have fallocate() on Linux

Commit a596f047 broke build en eg CentOS 5:

filesetup.c:17:26: error: linux/falloc.h: No such file or directory
filesetup.c: In function 'extend_file':
filesetup.c:95: warning: implicit declaration of function 'fallocate'
filesetup.c:95: error: 'FALLOC_FL_KEEP_SIZE' undeclared (first use in this function)
filesetup.c:95: error: (Each undeclared identifier is reported only once
filesetup.c:95: error: for each function it appears in.)
make: *** [filesetup.o] Error 1

Signed-off-by: Jens Axboe <jaxboe@fusionio.com>