Include sys/wait.h everywhere WIFEXITED etc is used

This is required on NetBSD, and I think FreeBSD too.

src/libfetchers/git.cc

@@ -6,6 +6,7 @@
 #include "url-parts.hh"
 
 #include <sys/time.h>
+#include <sys/wait.h>
 
 using namespace std::string_literals;
 

src/libstore/build/derivation-goal.cc

@@ -20,6 +20,7 @@
 #include <sys/types.h>
 #include <sys/socket.h>
 #include <sys/un.h>
+#include <sys/wait.h>
 #include <netdb.h>
 #include <fcntl.h>
 #include <termios.h>