Resolve linker issues with Android API 21 (dprintf, tcdrain)

Signed-off-by: Chris Renshaw <osm0sis@outlook.com>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>

libbb/missing_syscalls.c

@@ -40,8 +40,10 @@ int pivot_root(const char *new_root, const char *put_old)
 	return syscall(__NR_pivot_root, new_root, put_old);
 }
 
+# if __ANDROID_API__ < 21
 int tcdrain(int fd)
 {
 	return ioctl(fd, TCSBRK, 1);
 }
+# endif
 #endif