re-added signal.h

mongoose.c

@@ -100,6 +100,7 @@ typedef SOCKET sock_t;
 #include <fcntl.h>
 #include <netdb.h>
 #include <pthread.h>
+#include <signal.h>
 #include <stdarg.h>
 #include <unistd.h>
 #include <arpa/inet.h>  // For inet_pton() when NS_ENABLE_IPV6 is defined