Fix EXTRA_DIST to include all $(POTFILES)

Some files such as gnutls_tpm2_ibm.c were not being included in the
tarball.

Signed-off-by: David Woodhouse <dwmw2@infradead.org>

Makefile.am

@@ -143,7 +143,7 @@ endif
 
 pkgconfig_DATA = openconnect.pc
 
-EXTRA_DIST = AUTHORS version.sh COPYING.LGPL openconnect.ico $(lib_srcs_openssl) $(lib_srcs_gnutls) openconnect.nsi.in
+EXTRA_DIST = AUTHORS version.sh COPYING.LGPL openconnect.ico $(POTFILES) openconnect.nsi.in
 EXTRA_DIST += $(shell cd "$(top_srcdir)" && \
 		git ls-tree HEAD -r --name-only -- android/ java/ trojans/ bash/ 2>/dev/null)