build: change oldincludedir to includedir

Change-Id: I070d49e5ea488547eaded4d88b468c8eb85641f3
Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
diff --git a/Makefile.am b/Makefile.am
index 70b4895..7f99e00 100644
--- a/Makefile.am
+++ b/Makefile.am
@@ -11,7 +11,7 @@
 libmapper_la_LDFLAGS = $(SYSTEMD_LIBS) -version-info 1:0:0 -shared
 libmapper_la_CFLAGS = $(SYSTEMD_CFLAGS)
 
-oldinclude_HEADERS = libmapper/mapper.h
+include_HEADERS = libmapper/mapper.h
 
 if HAVE_PYTHON