[goffice: 1/2] Add missing string.h include

[Morten Welinder <mortenw src gnome org>]

commit 54098b6cd7dddc573b937f45048dd8a434f581af
Author: Gilles Dartiguelongue <eva gentoo org>
Date:   Sat Nov 17 16:30:30 2018 +0100

    Add missing string.h include

 tests/test-quad.c | 1 +
 1 file changed, 1 insertion(+)
---
diff --git a/tests/test-quad.c b/tests/test-quad.c
index 49d751e8..379d91ed 100644
--- a/tests/test-quad.c
+++ b/tests/test-quad.c
@@ -1,3 +1,4 @@
+#include <string.h>
 #include <goffice/goffice.h>
 
 // A rather expensive way of forcing the compiler to drop any excess