conf: include signal.h for str2sig() and SIGHUP

The reload-signal translation calls str2sig() and compares against
SIGHUP, but conf.c never included signal.h.  glibc pulls it in
transitively, so the omission went unnoticed until a cross-compile
against uClibc-ng in Buildroot failed to build.

Signed-off-by: Joachim Wiberg <troglobit@gmail.com>

src/conf.c

@@ -55,6 +55,7 @@
 # include <lite/queue.h>	/* BSD sys/queue.h API */
 #endif
 #include <confuse.h>
+#include <signal.h>
 #include <time.h>
 #include <glob.h>