Fix a build error

When we try to build libevhtp for the first time, we will get an build
error that indicates that SIGTERM is undeclared.  So fix it.

test_proxy.c

@@ -4,6 +4,7 @@
 #include <stdint.h>
 #include <errno.h>
 #include <evhtp.h>
+#include <signal.h>
 
 
 int