build: fix mingw build

Commit 02c39a2 introduced a mingw build regression, due to a
regression in gnulib's areadlink module:

../../../gnulib/lib/careadlinkat.c: In function 'careadlinkat':
../../../gnulib/lib/careadlinkat.c:143:39: error: 'const struct allocator' has no member named 'malloc'

* .gnulib: Update to latest, for careadlinkat fix.

diff --git a/.gnulib b/.gnulib
index bd5d1e621103ea3f7fbae09010348ce2df563808..9cc991025d6139a3a8f3e0f1570bf39a66f3aafa 160000 (submodule)
--- a/.gnulib
+++ b/.gnulib
@@ -1 +1 @@
-Subproject commit bd5d1e621103ea3f7fbae09010348ce2df563808
+Subproject commit 9cc991025d6139a3a8f3e0f1570bf39a66f3aafa