Move sigrtmin.h into platform headers.

Test: mma
Change-Id: Ibc97ef96fe9e32b948c245b06039b95fe4342f39
diff --git a/libc/bionic/sigaction.cpp b/libc/bionic/sigaction.cpp
index 96e6f3c..583bf32 100644
--- a/libc/bionic/sigaction.cpp
+++ b/libc/bionic/sigaction.cpp
@@ -29,7 +29,7 @@
 #include <signal.h>
 #include <string.h>
 
-#include "private/sigrtmin.h"
+#include <platform/bionic/reserved_signals.h>
 
 extern "C" void __restore_rt(void);
 extern "C" void __restore(void);