sparse: avoid warnings about known cURL issues in gvfs-helper.c

`sparse` complains with an error message like this:

	gvfs-helper.c:2912:17: error: expression using sizeof on a
	function

The culprit is this line:

	curl_easy_setopt(slot->curl, CURLOPT_WRITEFUNCTION, fwrite);

Similar lines exist in `http-push.c` and other files that are in
upstream Git, and to avoid these bogus warnings, they are already
exempted from `sparse`'s tender, loving care. We simply add
`gvfs-helper.c` to that list.

Signed-off-by: Derrick Stolee <dstolee@microsoft.com>

Makefile

@@ -2807,7 +2807,7 @@ gettext.sp gettext.s gettext.o: GIT-PREFIX
 gettext.sp gettext.s gettext.o: EXTRA_CPPFLAGS = \
 	-DGIT_LOCALE_PATH='"$(localedir_relative_SQ)"'
 
-http-push.sp http.sp http-walker.sp remote-curl.sp imap-send.sp: SP_EXTRA_FLAGS += \
+http-push.sp http.sp http-walker.sp remote-curl.sp imap-send.sp gvfs-helper.sp: SP_EXTRA_FLAGS += \
 	-DCURL_DISABLE_TYPECHECK
 
 pack-revindex.sp: SP_EXTRA_FLAGS += -Wno-memcpy-max-count