Fix Makefile of example

Use installed binary (of ecppll) in $PATH,
not the leftover from the build inside the repo with relative path

examples/ulx3s/Makefile

diff --git a/examples/ulx3s/Makefile b/examples/ulx3s/Makefile
index fc251c4..24bef3a 100644
--- a/examples/ulx3s/Makefile
+++ b/examples/ulx3s/Makefile
@@ -30,7 +30,7 @@
 	ujprog -t -b 3000000 $<
 
 pll_%.v:
-	../../libtrellis/ecppll \
+	ecppll \
 		-i 25 \
 		-o $(subst pll_,,$(basename $@)) \
 		-n $(basename $@) \