Fix compiling libprocessgroup against musl

Include sys/cdefs.h before using __BEGIN_DECLS.

Bug: 190084016
Test: m USE_HOST_MUSL=true
Change-Id: Iac009275f7198476dd83ac70dbbc626b70f88ed1
diff --git a/libprocessgroup/cgrouprc/include/android/cgrouprc.h b/libprocessgroup/cgrouprc/include/android/cgrouprc.h
index 100d60e..e704a36 100644
--- a/libprocessgroup/cgrouprc/include/android/cgrouprc.h
+++ b/libprocessgroup/cgrouprc/include/android/cgrouprc.h
@@ -16,6 +16,7 @@
 
 #pragma once
 
+#include <sys/cdefs.h>
 #include <stdint.h>
 
 __BEGIN_DECLS