Make unified headers' SIGRTMIN/SIGRTMAX usable before API 21.

Bug: https://github.com/android-ndk/ndk/issues/352
Test: built new NDK test
Change-Id: Iacebe574bbf693701949e038005a40ba6520d592
diff --git a/libc/bionic/__libc_current_sigrtmax.cpp b/libc/bionic/__libc_current_sigrtmax.cpp
index 27fcb35..32179bb 100644
--- a/libc/bionic/__libc_current_sigrtmax.cpp
+++ b/libc/bionic/__libc_current_sigrtmax.cpp
@@ -29,5 +29,7 @@
 #include <signal.h>
 
 int __libc_current_sigrtmax(void) {
+  // If you change this, also change __ndk_legacy___libc_current_sigrtmax
+  // in <android/legacy_signal_inlines.h> to match.
   return __SIGRTMAX;
 }