ffado: fix build with glibc-2.26

see https://sourceforge.net/p/ffado/mailman/message/36008067/

pkgs/os-specific/linux/ffado/default.nix

@@ -41,7 +41,10 @@ stdenv.mkDerivation rec {
     optXdg_utils libxmlxx glibmm
   ];
 
-  patches = [ ./gcc6.patch ];
+  patches = [
+    ./gcc6.patch
+    ./glibc226.patch
+  ];
 
   postPatch = ''
     # SConstruct checks cpuinfo and an objdump of /bin/mount to determine the appropriate arch

pkgs/os-specific/linux/ffado/glibc226.patch

@@ -0,0 +1,10 @@
+--- libffado/src/libutil/PosixMessageQueue.cpp  (revision 2705)
++++ libffado/src/libutil/PosixMessageQueue.cpp  (revision 2706)
+@@ -30,6 +30,7 @@
+ #include <errno.h>
+ #include <string.h>
+ #include <poll.h>
++#include <signal.h>
+
+ #define MQ_INVALID_ID ((mqd_t) -1)
+ // one second