Adapt the _ALT style to our new standard

- in .h files: only put the context declaration inside the #ifdef _ALT
  (this was changed in 2.9.0, ie after the original PR)
- in .c file: only leave selftest out of _ALT: even though some function are
  trivial to build from other parts, alt implementors might want to go another
way about them (for efficiency or other reasons)

include/mbedtls/poly1305.h

@@ -32,10 +32,10 @@
 #include <stdint.h>
 #include <stddef.h>
 
-#if !defined(MBEDTLS_POLY1305_ALT)
-
 #define MBEDTLS_ERR_POLY1305_BAD_INPUT_DATA -0x0041 /**< Invalid input parameter(s). */
 
+#if !defined(MBEDTLS_POLY1305_ALT)
+
 typedef struct
 {
     uint32_t r[4];      /** Stores the value for 'r' (low 128 bits of the key) */
@@ -46,6 +46,10 @@ typedef struct
 }
 mbedtls_poly1305_context;
 
+#else  /* MBEDTLS_POLY1305_ALT */
+#include "poly1305_alt.h"
+#endif /* MBEDTLS_POLY1305_ALT */
+
 /**
  * \brief               Initialize a Poly1305 context
  *
@@ -109,10 +113,6 @@ int mbedtls_poly1305_update( mbedtls_poly1305_context *ctx,
 int mbedtls_poly1305_finish( mbedtls_poly1305_context *ctx,
                              unsigned char mac[16] );
 
-#else  /* MBEDTLS_POLY1305_ALT */
-#include "poly1305_alt.h"
-#endif /* MBEDTLS_POLY1305_ALT */
-
 /**
  * \brief               Generate the Poly1305 MAC of some data with the given key.
  *