Makefile: Disable gettext in the Git submodule

Makefile: Disable gettext in the Git submodule

Newer libgit versions depend on the libintl library. However, we
currently do not link against libintl which breaks compilation under
OpenBSD:

    git/libgit.a(commit.o)(.text+0x1d1b): In function `lookup_commit_or_die':
    git/gettext.h:47: undefined reference to `libintl_gettext'
    [...]

Since we do not support i18n in cgit, just disable gettext in the Git
submodule to fix this.

Signed-off-by: Lukas Fleischer <cgit@cryptocrack.de>