Fix btio.c compilation warning

This patch fixes a compilation warning regarding btio/btio.c. Actually
this warning seems a false positive by Ubuntu Natty GCC version. A new
bug on Ubuntu bug system was opened but if you do not want to wait until
it is analyzed, just apply this minor fix.

btio/btio.c: In function 'bt_io_get':
btio/btio.c:803:11: warning: 'flushable' may be used uninitialized in
this function
1 file changed