extras/Makefile.am: fix libwebpextras.la reference

drop './' from the reference in webp_quality_LDADD.. this form is used
in the other makefiles. this fixes a parallel build failure seen under
freebsd:
make[1]: don't know how to make ./libwebpextras.la. Stop

Change-Id: I59635a0c747d402cd990f6379eb1948c3e40f278
1 file changed