plugin/webp: autotools: add linking to gvc

plugin/webp: autotools: add linking to gvc

E.g. plugin/webp/gvloadimage_webp.c used gvusershape_file_access from
lib/gvc/gvusershape.c.

Fixes the following errors with Cygwin:

gvloadimage_webp.o: in function `webp_loadimage':
plugin/webp/gvloadimage_webp.c:139: undefined reference to `__imp_gvusershape_file_access'
plugin/webp/gvloadimage_webp.c:153: undefined reference to `__imp_gvusershape_file_release'
plugin/webp/gvloadimage_webp.c:153: undefined reference to `__imp_gvusershape_file_release'
gvdevice_webp.o: in function `writer':
plugin/webp/gvdevice_webp.c:43: undefined reference to `gvwrite'