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/pem.h b/include/mbedtls/pem.h
index 2cf4c0a..fa82f7b 100644
--- a/include/mbedtls/pem.h
+++ b/include/mbedtls/pem.h
@@ -51,7 +51,7 @@
 /**
  * \brief       PEM context structure
  */
-typedef struct
+typedef struct mbedtls_pem_context
 {
     unsigned char *buf;     /*!< buffer for decoded data             */
     size_t buflen;          /*!< length of the buffer                */