Include cdefs-compat.h on s390 and powerpc

They use __BEGIN_DECLS which might not be avaiable without cdefs-compat.h

include/openlibm_fenv_powerpc.h

@@ -31,6 +31,7 @@
 
 #include <stdint.h>
 #include <sys/types.h>
+#include "cdefs-compat.h"
 
 #ifndef	__fenv_static
 #define	__fenv_static	static

include/openlibm_fenv_s390.h

@@ -31,6 +31,7 @@
 
 #include <stdint.h>
 #include <sys/types.h>
+#include "cdefs-compat.h"
 
 #ifndef	__fenv_static
 #define	__fenv_static	static