Fix failure to maintainer-clean jsonpath_gram.h

Oversight in dac048f

Author: j-naylor

src/backend/Makefile

@@ -306,6 +306,7 @@ maintainer-clean: distclean
 	      storage/lmgr/lwlocknames.c \
 	      storage/lmgr/lwlocknames.h \
 	      utils/adt/jsonpath_gram.c \
+	      utils/adt/jsonpath_gram.h \
 	      utils/adt/jsonpath_scan.c \
 	      utils/misc/guc-file.c