[vala] posix: add feature_test_macro _GNU_SOURCE to sighandler_t

[Dr. Michael Lauer <mlauer src gnome org>]

commit 32330196c74b66714b1607970d1059fbef533235
Author: Dr. Michael Lauer <mickey vanille-media de>
Date:   Wed Feb 21 10:21:29 2018 +0100

    posix: add feature_test_macro _GNU_SOURCE to sighandler_t
    
    https://bugzilla.gnome.org/show_bug.cgi?id=761975

 vapi/posix.vapi |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)
---
diff --git a/vapi/posix.vapi b/vapi/posix.vapi
index 1274a2b..9ea15ed 100644
--- a/vapi/posix.vapi
+++ b/vapi/posix.vapi
@@ -1259,7 +1259,7 @@ namespace Posix {
        [CCode (cheader_filename = "signal.h")]
        public int sigaction (int signum, sigaction_t? act, out sigaction_t? oldact);
 
-       [CCode (has_target = false, cheader_filename = "signal.h")]
+       [CCode (has_target = false, cheader_filename = "signal.h", feature_test_macro = "_GNU_SOURCE")]
        public delegate void sighandler_t (int signal);
 
        [CCode (has_target = false, cheader_filename = "signal.h")]