maint: Fix VPATH build

In a VPATH build, <config.h> is in the builddir (which automake
includes automatically), but it includes <config-post.h> from the
top source directory (which is not automatic); hence, we need to
keep the -I(top_srcdir) directive that was accidentally removed
from commit 7a879323 (the problem is not visible in an in-tree
build).
Signed-off-by: NEric Blake <eblake@redhat.com>

gnulib/lib/Makefile.am

@@ -26,3 +26,5 @@ SUFFIXES =
 noinst_LTLIBRARIES =
 
 include gnulib.mk
+
+AM_CPPFLAGS = -I$(top_srcdir)