Reserve realtime signal for libprofile-extras

Bug: http://b/128524141

This signal is used by libprofile-extras to trigger flush of gcov
coverage data.

Test: libprofile-extras in system/extras/toolchain-extras
Change-Id: I77dd5b1d67371a9cfaad54ad60254bfb2dd18ba9
diff --git a/libc/include/android/legacy_signal_inlines.h b/libc/include/android/legacy_signal_inlines.h
index 8219759..5ca9813 100644
--- a/libc/include/android/legacy_signal_inlines.h
+++ b/libc/include/android/legacy_signal_inlines.h
@@ -52,7 +52,7 @@
 
 static __inline int __ndk_legacy___libc_current_sigrtmin() {
   if (__libc_current_sigrtmin) return __libc_current_sigrtmin();
-  return __SIGRTMIN + 5; /* Should match __libc_current_sigrtmin. */
+  return __SIGRTMIN + 6; /* Should match __libc_current_sigrtmin. */
 }
 
 #undef SIGRTMAX