[libgxps] Fix build (with GCC 6?)

[Michael Catanzaro <mcatanzaro src gnome org>]

commit 3fb36de95522bf637b238b73f1587dfc5caf9fd7
Author: Michael Catanzaro <mcatanzaro gnome org>
Date:   Tue May 17 06:04:22 2016 -0500

    Fix build (with GCC 6?)
    
    This is a test case, so no reason to worry about format nonliterals.
    (Are these errors by default now?)

 test/test-gxps.c |    3 +++
 1 files changed, 3 insertions(+), 0 deletions(-)
---
diff --git a/test/test-gxps.c b/test/test-gxps.c
index 5015447..5f2e77c 100644
--- a/test/test-gxps.c
+++ b/test/test-gxps.c
@@ -88,7 +88,10 @@ format_date (time_t utime)
         struct tm *t;
         if (time == 0 || !(t = localtime (&time)) )
                 return NULL;
+#pragma GCC diagnostic push
+#pragma GCC diagnostic ignored "-Wformat-nonliteral"
         len = strftime (s, sizeof (s), fmt_hack, t);
+#pragma GCC diagnostic pop
 #endif
 
         if (len == 0 || s[0] == '\0')