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)
diff --git a/include/mbedtls/chacha20.h b/include/mbedtls/chacha20.h
index ccce122..a2856a7 100644
--- a/include/mbedtls/chacha20.h
+++ b/include/mbedtls/chacha20.h
@@ -31,13 +31,13 @@
 #include MBEDTLS_CONFIG_FILE
 #endif
 
-#if !defined(MBEDTLS_CHACHA20_ALT)
-
 #include <stdint.h>
 #include <stddef.h>
 
 #define MBEDTLS_ERR_CHACHA20_BAD_INPUT_DATA -0x003B /**< Invalid input parameter(s). */
 
+#if !defined(MBEDTLS_CHACHA20_ALT)
+
 typedef struct
 {
     uint32_t initial_state[16];  /*! Holds the initial state (before round operations) */
@@ -47,6 +47,10 @@
 }
 mbedtls_chacha20_context;
 
+#else  /* MBEDTLS_CHACHA20_ALT */
+#include "chacha20_alt.h"
+#endif /* MBEDTLS_CHACHA20_ALT */
+
 /**
  * \brief           Initialize ChaCha20 context
  *
@@ -149,10 +153,6 @@
                               const unsigned char *input,
                               unsigned char *output );
 
-#else  /* MBEDTLS_CHACHA20_ALT */
-#include "chacha20_alt.h"
-#endif /* MBEDTLS_CHACHA20_ALT */
-
 /**
  * \brief           Encrypt or decrypt a message using ChaCha20.
  *