makefile: fail rpc-check after finding any changes

`rpc-check` is used in our CI pipeline to detect any difference
between the committed and the automatically generated rpc files.
Unfortunately, the current method only detects changes in the
already existing files and won't fail if `make rpc` generates
a totally new file.

`git status --porcelain` makes the trick, it returns a line for
each file that has been modified, created or deleted.

docs/release-notes/release-notes-0.15.0.md

@@ -83,6 +83,9 @@
   allows code external to lnd to call the function, where previously it would 
   require access to lnd's internals.
 
+* [rpc-check fails if it finds any changes](https://github.com/lightningnetwork/lnd/pull/6207/)
+  including new and deleted files.
+
 # Contributors (Alphabetical Order)
 
 * 3nprob
@@ -96,6 +99,7 @@
 * ErikEk
 * henta
 * Joost Jager
+* Jordi Montes
 * LightningHelper
 * Liviu
 * mateuszmp