Fix Coverity ID 1093805 Uninitialized scalar variable

1st use of fbfire_info.bfi_flags so assign instead of |=

git-svn-id: svn://svn.icculus.org/fs2open/trunk/fs2_open@10957 387891d4-d844-0410-90c0-e4c51a9137d3
Doh! Assign, not equality!

git-svn-id: svn://svn.icculus.org/fs2open/trunk/fs2_open@10958 387891d4-d844-0410-90c0-e4c51a9137d3