Include in parentselectdefs.h in install target (#7713)

Required now that ts/ts.h includes it (#7467)
diff --git a/include/ts/Makefile.am b/include/ts/Makefile.am
index b53837e..f59de4e 100644
--- a/include/ts/Makefile.am
+++ b/include/ts/Makefile.am
@@ -22,7 +22,8 @@
 	apidefs.h \
 	ts.h \
 	remap.h \
-	experimental.h
+	experimental.h \
+	parentselectdefs.h
 
 noinst_HEADERS = \
 	InkAPIPrivateIOCore.h