Move declarations of xmpp_{v,}snprintf.

Use HAVE_SNPRINTF and HAVE_VSNPRINTF to conditionnaly use an internal
version of these functions or use the libc one.

src/common.h

@@ -28,6 +28,7 @@
 #include "util.h"
 #include "parser.h"
 #include "rand.h"
+#include "snprintf.h"
 
 /** run-time context **/
 
@@ -266,8 +267,4 @@ void disconnect_mem_error(xmpp_conn_t * const conn);
 void auth_handle_open(xmpp_conn_t * const conn);
 void auth_handle_component_open(xmpp_conn_t * const conn);
 
-/* replacement snprintf and vsnprintf */
-int xmpp_snprintf (char *str, size_t count, const char *fmt, ...);
-int xmpp_vsnprintf (char *str, size_t count, const char *fmt, va_list arg);
-
 #endif /* __LIBSTROPHE_COMMON_H__ */