build: add missing header

Fix failing distcheck
diff --git a/Makefile.am b/Makefile.am
index 76262bb..545e90d 100644
--- a/Makefile.am
+++ b/Makefile.am
@@ -53,6 +53,7 @@
 	shared/hash.c \
 	shared/hash.h \
 	shared/scratchbuf.c \
+	shared/scratchbuf.h \
 	shared/strbuf.c \
 	shared/strbuf.h \
 	shared/util.c \