sim: Use extern for these structs to generate a linkage

Fixes:
duplicate symbol _ts102221_fcp_vals in:
    .libs/core.o
    .libs/card_fs_usim.o
duplicate symbol _pcsc_reader_ops in:
    .libs/reader.o
    .libs/card_fs_usim.o
diff --git a/src/sim/sim_int.h b/src/sim/sim_int.h
index c10c5f0..7024743 100644
--- a/src/sim/sim_int.h
+++ b/src/sim/sim_int.h
@@ -36,6 +36,6 @@
 	int (*transceive)(struct osim_reader_hdl *rh, struct msgb *msg);
 };
 
-const struct osim_reader_ops pcsc_reader_ops;
+extern const struct osim_reader_ops pcsc_reader_ops;
 
 #endif