Use the 'gnu_printf' format attribute

Running 'R CMD check git2r' gives the following warning on Windows

using R version 3.1.1 (2014-07-10)
* using platform: i386-w64-mingw32 (32-bit)
* checking whether package 'git2r' can be installed ... WARNING
Found the following significant warnings:
  libgit2/config.c:612:2: warning: too many arguments for format [-Wformat-extra-args]

* src/libgit2/win32/posix.h (p_snprintf): Changed the attribute
  from
    GIT_FORMAT_PRINTF(3, 4);
  to
    __attribute__((__format__ (gnu_printf, 3, 4)));
  to fix the warning

* misc/posix-pass-R-CMD-check-git2r.patch: Add

* Makefile: Update target 'sync_libgit2' to apply patch

Signed-off-by: Stefan Widgren <stefan.widgren@gmail.com>