allow to use <sys/ucontext.h>

For architectures without ucontext implementation it is possible
to use libucontext with this small adaptation.

include/ucontext.h

@@ -23,11 +23,11 @@
 
 #include <features.h>
 
-#ifdef __UCLIBC_HAS_CONTEXT_FUNCS__
-
 /* Get machine dependent definition of data structures.  */
 #include <sys/ucontext.h>
 
+#ifdef __UCLIBC_HAS_CONTEXT_FUNCS__
+
 __BEGIN_DECLS
 
 /* Get user context and store it in variable pointed to by UCP.  */