radeon: Fix build.

radeon: Fix build.

Fix build error introduced with commit
dfa25ea5cd19d5a050a1c94bd7370a2259b9f007.

  CC       r600_streamout.lo
r600_streamout.c:108:6: error: conflicting types for 'r600_set_streamout_targets'
void r600_set_streamout_targets(struct pipe_context *ctx,
     ^
./r600_pipe_common.h:413:6: note: previous declaration is here
void r600_set_streamout_targets(struct pipe_context *ctx,
     ^

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=76009
Signed-off-by: Vinson Lee <vlee@freedesktop.org>