Fix build when --disable-threads is used.

src/common/mythread.h

diff --git a/src/common/mythread.h b/src/common/mythread.h
index 7a68da8..be22654 100644
--- a/src/common/mythread.h
+++ b/src/common/mythread.h
@@ -81,6 +81,8 @@
 
 #if !(defined(_WIN32) && !defined(__CYGWIN__))
 // Use sigprocmask() to set the signal mask in single-threaded programs.
+#include <signal.h>
+
 static inline void
 mythread_sigmask(int how, const sigset_t *restrict set,
 		sigset_t *restrict oset)