Add missing include, required on some platforms

Noted by the buildfarm and Andres Freund

src/bin/pg_basebackup/pg_basebackup.c

@@ -22,6 +22,7 @@
 #include <sys/stat.h>
 #include <sys/types.h>
 #include <sys/wait.h>
+#include <signal.h>
 #include <time.h>
 
 #ifdef HAVE_LIBZ