Fixe compilation warning

Relevant output of make:

sgsnemu.c: In function ‘process_options’:
sgsnemu.c:443: warning: pointer targets in passing argument 1 of
‘strncpy’ differ in signedness
/usr/include/bits/string3.h:120: note: expected ‘char * __restrict__’
but argument is of type ‘unsigned char *’

Signed-off-by: Emmanuel Bretelle <chantra@debuntu.org>
diff --git a/sgsnemu/sgsnemu.c b/sgsnemu/sgsnemu.c
index 9b5c2e2..ead8839 100644
--- a/sgsnemu/sgsnemu.c
+++ b/sgsnemu/sgsnemu.c
@@ -440,7 +440,7 @@
     return -1;
   }
   options.apn.l = strlen(args_info.apn_arg);
-  strncpy(options.apn.v, args_info.apn_arg, sizeof(options.apn.v));
+  strncpy((char *)options.apn.v, args_info.apn_arg, sizeof(options.apn.v));
   options.apn.v[sizeof(options.apn.v)-1] = 0;
   printf("Using APN:             %s\n", args_info.apn_arg);