Fix builds when config.h only defines MBEDTLS_BIGNUM_C

Fixes #4929

Signed-off-by: Tom Cosgrove <tom.cosgrove@arm.com>
diff --git a/include/mbedtls/entropy.h b/include/mbedtls/entropy.h
index ac977cf..eea786e 100644
--- a/include/mbedtls/entropy.h
+++ b/include/mbedtls/entropy.h
@@ -157,7 +157,7 @@
                               * -1 after free. */
 #if defined(MBEDTLS_ENTROPY_SHA512_ACCUMULATOR)
     mbedtls_sha512_context  accumulator;
-#else
+#elif defined(MBEDTLS_ENTROPY_SHA256_ACCUMULATOR)
     mbedtls_sha256_context  accumulator;
 #endif
     int             source_count; /* Number of entries used in source. */