Makefile: dependencies for vcs-svn tests

Makefile: dependencies for vcs-svn tests

The vcs-svn tests (test-treap.o et al) depend on the vcs-svn
headers for declarations and inline functions.  Declare the
dependency.  While at it, declare a dependency of the vcs-svn
objects (vcs-svn/string_pool.o et al) on $(LIB_H) to reflect use
within the vcs-svn library of git-compat-util.h and cache.h.

Without this change, tweaks to inline functions in those headers
do not provoke rebuilds of the corresponding tests[*], making
such changes unnecessarily difficult to test.

Before:

 $ touch vcs-svn/*.h && make test-treap
 $

After:

 $ touch vcs-svn/*.h && make test-treap
 CC test-treap.o
 LINK test-treap
 $

[*] unless COMPUTE_HEADER_DEPENDENCIES is enabled

Detected with "make CHECK_HEADER_DEPENDENCIES=1".

Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>