PPC32: only define cond_syscall if it isn't already defined.

include/asm-ppc/unistd.h

@@ -461,7 +461,9 @@ extern pid_t waitpid(pid_t pid, int *wait_stat, int options);
  * What we want is __attribute__((weak,alias("sys_ni_syscall"))),
  * but it doesn't work on all toolchains, so we just do it by hand
  */
+#ifndef cond_syscall
 #define cond_syscall(x) asm(".weak\t" #x "\n\t.set\t" #x ",sys_ni_syscall");
+#endif
 
 #endif /* __KERNEL__ */