lmbench_usage() needs a prototype

libc no longer included RPC support. Therefore, need to change it to
handle where the header files have been moved to.

src/bench.h

@@ -35,9 +35,19 @@ typedef unsigned char bool_t;
 #include        <sys/un.h>
 #include        <sys/resource.h>
 #define PORTMAP
+#ifndef __linux__
 #include	<rpc/rpc.h>
+#else
+#include	<tirpc/rpc/rpc.h>
+#endif
+
 #endif
+
+#ifndef __linux__
 #include	<rpc/types.h>
+#else
+#include	<tirpc/rpc/types.h>
+#endif
 
 #include 	<stdarg.h>
 #ifndef HAVE_uint
@@ -260,6 +270,8 @@ extern	int	optopt;
 extern	char	*optarg;
 int	getopt(int ac, char **av, char *opts);
 
+void lmbench_usage(int argc, char *argv[], char* usage);
+
 typedef u_long iter_t;
 typedef void (*benchmp_f)(iter_t iterations, void* cookie);