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>

src/xkb/Makefile.am

@@ -6,6 +6,7 @@ AM_CPPFLAGS= \
 	-I$(top_srcdir)/src/xcms \
 	-I$(top_srcdir)/src/xkb \
 	-I$(top_srcdir)/src/xlibi18n \
+	-I$(top_srcdir)/src \
 	-D_BSD_SOURCE
 
 AM_CFLAGS= \