sparse: Fix some "symbol not declared" warnings

sparse: Fix some "symbol not declared" warnings

In particular, sparse issues the "symbol 'a_symbol' was not declared.
Should it be static?" warnings for the following symbols:

    attr.c:468:12: 'git_etc_gitattributes'
    attr.c:476:5:  'git_attr_system'
    vcs-svn/svndump.c:282:6: 'svndump_read'
    vcs-svn/svndump.c:417:5: 'svndump_init'
    vcs-svn/svndump.c:432:6: 'svndump_deinit'
    vcs-svn/svndump.c:445:6: 'svndump_reset'

The symbols in attr.c only require file scope, so we add the static
modifier to their declaration.

The symbols in vcs-svn/svndump.c are external symbols, and they
already have extern declarations in the "svndump.h" header file,
so we simply include the header in svndump.c.

Signed-off-by: Ramsay Jones <ramsay@ramsay1.demon.co.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>