Fix server header file installation with vpath builds

Several server header files would not be installed in vpath builds
because they live in the build directory.

src/include/Makefile

@@ -48,6 +48,12 @@ install: all installdirs
 	  cp $(srcdir)/$$dir/*.h '$(DESTDIR)$(includedir_server)'/$$dir/ || exit; \
 	  chmod $(INSTALL_DATA_MODE) '$(DESTDIR)$(includedir_server)'/$$dir/*.h  || exit; \
 	done
+ifeq ($(vpath_build),yes)
+	for file in dynloader.h parser/gram.h utils/probes.h; do \
+	  cp $$file '$(DESTDIR)$(includedir_server)'/$$file || exit; \
+	  chmod $(INSTALL_DATA_MODE) '$(DESTDIR)$(includedir_server)'/$$file || exit; \
+	done
+endif
 
 installdirs:
 	$(mkinstalldirs) '$(DESTDIR)$(includedir)/libpq' '$(DESTDIR)$(includedir_internal)/libpq'