Makefile: add missing hpl/rtc directory

Change-Id: I3fed159173467d1da091758fe2a87bef2cd2e3c7
diff --git a/sysmoOCTSIM/gcc/Makefile b/sysmoOCTSIM/gcc/Makefile
index 97c5ec1..01088cc 100644
--- a/sysmoOCTSIM/gcc/Makefile
+++ b/sysmoOCTSIM/gcc/Makefile
@@ -63,6 +63,7 @@
 	hpl/pm \
 	hpl/ramecc \
 	hpl/sercom \
+	hpl/rtc \
 	hpl/usb \
 	stdio_redirect \
 	stdio_redirect/gcc \