Makefile: quote $INSTLIBDIR when passing it to sed

Makefile: quote $INSTLIBDIR when passing it to sed

f6a0ad4b (Makefile: generate Perl header from template file,
2018-04-10) moved code for generating the 'use lib' lines at the top
of perl scripts from the $(SCRIPT_PERL_GEN) rule to a separate
GIT-PERL-HEADER rule.

This rule first populates INSTLIBDIR and then substitutes it into the
GIT-PERL-HEADER using sed:

	INSTLIBDIR=... something ...
	sed -e 's=@@INSTLIBDIR@@='$$INSTLIBDIR'=g' $< > $@

Because $INSTLIBDIR is not surrounded by double quotes, the shell
splits it at each space, causing errors if INSTLIBDIR contains an $IFS
character:

 sed: 1: "s=@@INSTLIBDIR@@=/usr/l ...": unescaped newline inside substitute pattern

Add back the missing double-quotes to make it work again.

Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>