Install set_user.h to server include dir so .deb files for different PG versions do not conflict

--- a/Makefile
+++ b/Makefile
@@ -30,10 +30,10 @@ endif
 install: install-headers
 
 install-headers:
-	$(MKDIR_P) "$(DESTDIR)$(includedir)"
-	$(INSTALL_DATA) "src/set_user.h" "$(DESTDIR)$(includedir)"
+	$(MKDIR_P) "$(DESTDIR)$(includedir_server)"
+	$(INSTALL_DATA) "src/set_user.h" "$(DESTDIR)$(includedir_server)"
 
 uninstall: uninstall-headers
 
 uninstall-headers:
-	rm "$(DESTDIR)$(includedir)/set_user.h"
+	rm "$(DESTDIR)$(includedir_server)/set_user.h"
