Move sigrtmin.h into platform headers.

Test: mma
Change-Id: Ibc97ef96fe9e32b948c245b06039b95fe4342f39
diff --git a/libc/bionic/__libc_current_sigrtmin.cpp b/libc/bionic/__libc_current_sigrtmin.cpp
index d2ea75d..e6b3412 100644
--- a/libc/bionic/__libc_current_sigrtmin.cpp
+++ b/libc/bionic/__libc_current_sigrtmin.cpp
@@ -28,7 +28,7 @@
 
 #include <signal.h>
 
-#include "private/sigrtmin.h"
+#include <platform/bionic/reserved_signals.h>
 
 int __libc_current_sigrtmin() {
   return __SIGRTMIN + __SIGRT_RESERVED;