Pass the correct flag to find_symdef() from _rtld_bind().

When symbol versioning was added to rtld, the boolean 'in_plt' argument
to find_symdef() was converted to a bitmask of flags. The first flag
added was 'SYMLOOK_IN_PLT' which replaced the 'in_plt' bool. This
happened to still work by accident as SYMLOOK_IN_PLT had the value of 1
which is the same as 'true', so there should be no functional change.

Tested on: amd64
Reviewed by: kan
MFC after: 2 weeks
Sponsored by: DARPA / AFRL