[perf] fix typo in perf Makefile.

perf/Makefile.am

diff --git a/perf/Makefile.am b/perf/Makefile.am
index 78ca3ba..a28a3ef 100644
--- a/perf/Makefile.am
+++ b/perf/Makefile.am
@@ -4,7 +4,7 @@
 EXTRA_DIST =
 SUBDIRS =
 
-EXTRA_DIST +=
+EXTRA_DIST += \
 	meson.build \
 	benchmark-font.cc \
 	benchmark-map.cc \