Fix Solaris compile

In file included from fio.h:24,
                  from gettime.c:10:
os/os.h:192: error: redefinition of 'gettid'
os/os-solaris.h:123: error: previous definition of 'gettid' was here
make: *** [gettime.o] Error 1

Forgot to define FIO_HAVE_GETTID

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