Add `MBEDTLS_SELF_TEST` check in header files

Add the `MBEDTLS_SELF_TEST` precompilation surrounding the self test functions,
which were missing this check in the header files. ( most of the header files were missing this check).
Addresses issue #971
diff --git a/include/mbedtls/base64.h b/include/mbedtls/base64.h
index 7a64f52..9af1149 100644
--- a/include/mbedtls/base64.h
+++ b/include/mbedtls/base64.h
@@ -75,6 +75,7 @@
 int mbedtls_base64_decode( unsigned char *dst, size_t dlen, size_t *olen,
                    const unsigned char *src, size_t slen );
 
+#if defined(MBEDTLS_SELF_TEST)
 /**
  * \brief          Checkup routine
  *
@@ -82,6 +83,8 @@
  */
 int mbedtls_base64_self_test( int verbose );
 
+#endif /* MBEDTLS_SELF_TEST */
+
 #ifdef __cplusplus
 }
 #endif