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/sha1.h b/include/mbedtls/sha1.h
index 65a124c..88f0e8c 100644
--- a/include/mbedtls/sha1.h
+++ b/include/mbedtls/sha1.h
@@ -304,6 +304,8 @@
 #undef MBEDTLS_DEPRECATED
 #endif /* !MBEDTLS_DEPRECATED_REMOVED */
 
+#if defined(MBEDTLS_SELF_TEST)
+
 /**
  * \brief          The SHA-1 checkup routine.
  *
@@ -317,6 +319,8 @@
  */
 int mbedtls_sha1_self_test( int verbose );
 
+#endif /* MBEDTLS_SELF_TEST */
+
 #ifdef __cplusplus
 }
 #endif