Fix out-of-tree builds after commit 8a62e265

In-tree builds found reallocarray.h in $(top_builddir)/src but the
out-of-tree build didn't find it at all.

Reported-by: Emmanuele Bassi <ebassi@gmail.com> from GNOME continuous integration pipeline
Signed-off-by: Alan Coopersmith <alan.coopersmith@oracle.com>