Remove unused header files

* src/libgit2/win32/mingw-compat.h: Removed. Not needed when building
  with mingw-w64.

* src/libgit2/win32/msvc-compat.h: Removed. Not needed when building
  with mingw-w64.

* Makefile: Update target sync_libgit2 to remove mingw-compat.h and
  msvc-compat.h. Run patch to remove #include mingw-compat.h and
  #include msvc-compat.h from src/libgit2/common.h.

* patches/common-remove-includes.patch: Patch to remove #include
  mingw-compat.h and #include msvc-compat.h from src/libgit2/common.h.

* src/libgit2/common.h: Update after running patch

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

* src/libgit2/util.c: Remove unused function git__qsort_r_glue_cmp

* patches/util-pass-R-CMD-check-git2r.patch: Update patch to remove
  git__qsort_r_glue_cmp

Signed-off-by: Stefan Widgren <stefan.widgren@gmail.com>
Define file offset bits Windows

* src/Makevars.win: Define -D_FILE_OFFSET_BITS=64

* src/build_Makevars.r: Update to add definition to Makevars.win

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