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
diff --git a/extras/Makefile.am b/extras/Makefile.am
index 28ec20b..4863c61 100644
--- a/extras/Makefile.am
+++ b/extras/Makefile.am
@@ -22,5 +22,5 @@
 webp_quality_SOURCES  = webp_quality.c
 webp_quality_CPPFLAGS = $(AM_CPPFLAGS) $(USE_EXPERIMENTAL_CODE)
 webp_quality_LDADD  = ../imageio/libimageio_util.la
-webp_quality_LDADD += ./libwebpextras.la
+webp_quality_LDADD += libwebpextras.la
 webp_quality_LDADD += ../src/libwebp.la