Makefile: use VCSSVN_LIB to refer to svn library

We have an abstracted variable; let's use it consistently.
Signed-off-by: NJeff King <peff@peff.net>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

Makefile

@@ -2231,9 +2231,9 @@ perf: all
 
 .PHONY: test perf
 
-t/helper/test-line-buffer$X: vcs-svn/lib.a
+t/helper/test-line-buffer$X: $(VCSSVN_LIB)
 
-t/helper/test-svn-fe$X: vcs-svn/lib.a
+t/helper/test-svn-fe$X: $(VCSSVN_LIB)
 
 .PRECIOUS: $(TEST_OBJS)