config: Simplify incorrect GCM comment

GCM is not just for AES, but for at least Camellia as well.
diff --git a/include/mbedtls/config.h b/include/mbedtls/config.h
index 18a6097..8577365 100644
--- a/include/mbedtls/config.h
+++ b/include/mbedtls/config.h
@@ -1382,7 +1382,7 @@
 /**
  * \def MBEDTLS_GCM_C
  *
- * Enable the Galois/Counter Mode (GCM) for AES.
+ * Enable the Galois/Counter Mode (GCM).
  *
  * Module:  library/gcm.c
  *