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>
diff --git a/libbb/missing_syscalls.c b/libbb/missing_syscalls.c
index e3c1e92..0934128 100644
--- a/libbb/missing_syscalls.c
+++ b/libbb/missing_syscalls.c
@@ -40,8 +40,10 @@
 	return syscall(__NR_pivot_root, new_root, put_old);
 }
 
+# if __ANDROID_API__ < 21
 int tcdrain(int fd)
 {
 	return ioctl(fd, TCSBRK, 1);
 }
+# endif
 #endif