configure.ac: Add missing comma to CC_LD_DYNPATH

configure.ac: Add missing comma to CC_LD_DYNPATH

40bfbde ("build: don't duplicate substitution of make variables",
2012-09-11) by mistake removed a necessary comma at the end of
"CC_LD_DYNPATH=-Wl,rpath," in line 414.

When executing "./configure --with-zlib=PATH", this resulted in

      [...]
      CC xdiff/xhistogram.o
      AR xdiff/lib.a
      LINK git-credential-store
  /usr/bin/ld: bad -rpath option
  collect2: ld returned 1 exit status
  make: *** [git-credential-store] Error 1
  $

during make.

Signed-off-by: Øyvind A. Holm <sunny@sunbase.org>
Acked-by: Stefano Lattarini <stefano.lattarini@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>