Add ifdef for selftest in header file

See https://github.com/ARMmbed/mbedtls/pull/975
diff --git a/include/mbedtls/chacha20.h b/include/mbedtls/chacha20.h
index c33aef3..56ee57a 100644
--- a/include/mbedtls/chacha20.h
+++ b/include/mbedtls/chacha20.h
@@ -196,6 +196,7 @@
                             const unsigned char* input,
                             unsigned char* output );
 
+#if defined(MBEDTLS_SELF_TEST)
 /**
  * \brief           The ChaCha20 checkup routine.
  *
@@ -203,6 +204,7 @@
  * \return          \c 1 on failure.
  */
 int mbedtls_chacha20_self_test( int verbose );
+#endif /* MBEDTLS_SELF_TEST */
 
 #ifdef __cplusplus
 }
diff --git a/include/mbedtls/chachapoly.h b/include/mbedtls/chachapoly.h
index ae786e0..249dba1 100644
--- a/include/mbedtls/chachapoly.h
+++ b/include/mbedtls/chachapoly.h
@@ -311,6 +311,7 @@
                                      const unsigned char *input,
                                      unsigned char *output );
 
+#if defined(MBEDTLS_SELF_TEST)
 /**
  * \brief           The ChaCha20-Poly1305 checkup routine.
  *
@@ -318,6 +319,7 @@
  * \return          \c 1 on failure.
  */
 int mbedtls_chachapoly_self_test( int verbose );
+#endif /* MBEDTLS_SELF_TEST */
 
 #ifdef __cplusplus
 }
diff --git a/include/mbedtls/poly1305.h b/include/mbedtls/poly1305.h
index babbc15..abe3690 100644
--- a/include/mbedtls/poly1305.h
+++ b/include/mbedtls/poly1305.h
@@ -164,6 +164,7 @@
                           size_t ilen,
                           unsigned char mac[16] );
 
+#if defined(MBEDTLS_SELF_TEST)
 /**
  * \brief           The Poly1305 checkup routine.
  *
@@ -171,6 +172,7 @@
  * \return          \c 1 on failure.
  */
 int mbedtls_poly1305_self_test( int verbose );
+#endif /* MBEDTLS_SELF_TEST */
 
 #ifdef __cplusplus
 }