Allow to forward declare of public structs #1215

Thanks to forward declare we can declare `struct` in our header file instead making #include
diff --git a/include/mbedtls/gcm.h b/include/mbedtls/gcm.h
index 87535ab..d2098eb 100644
--- a/include/mbedtls/gcm.h
+++ b/include/mbedtls/gcm.h
@@ -53,7 +53,8 @@
 /**
  * \brief          The GCM context structure.
  */
-typedef struct {
+typedef struct mbedtls_gcm_context
+{
     mbedtls_cipher_context_t cipher_ctx;  /*!< The cipher context used. */
     uint64_t HL[16];                      /*!< Precalculated HTable low. */
     uint64_t HH[16];                      /*!< Precalculated HTable high. */