meson.build: quote the GITWEBDIR build configuration

meson.build: quote the GITWEBDIR build configuration

The build configuration options with (non-empty) values, for example
filesystem paths potentially containing spaces, have been set using
the '.set_quoted()' method. However, the GITWEBDIR value has been
set using the '.set()' method instead. In order to correctly quote
the GITWEBDIR value, replace the '.set()' method with '.set_quoted()'.

Signed-off-by: Ramsay Jones <ramsay@ramsayjones.plus.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>