Remove #error from include files.

Don't have #error statements in header files, but instead wrap
the contents of that file in #ifndef OPENSSL_NO_xxx
This means it is now always safe to include the header file.

Reviewed-by: Richard Levitte <levitte@openssl.org>

crypto/include/internal/poly1305.h

@@ -49,10 +49,6 @@
  */
 #include <stddef.h>
 
-#ifdef OPENSSL_NO_POLY1305
-# error POLY1305 is disabled.
-#endif
-
 #define POLY1305_BLOCK_SIZE 16
 
 typedef struct poly1305_context POLY1305;