Add missing -Isrc/ext to tor-fw-helper/include.am

We need this now that tor-fw-helper will pull in siphash.h

Fixes bug 11296; bugfix on 0.2.5.4-alpha where siphash.h was introduced.

changes/bug11296

@@ -0,0 +1,3 @@
+  o Minor bugfixes:
+    - Allow tor-fw-helper to build again by adding src/ext to its
+      CPPFLAGS. Fixes bug 11296; bugfix on 0.2.5.4-alpha.

src/tools/tor-fw-helper/include.am

@@ -33,4 +33,4 @@ endif
 
 src_tools_tor_fw_helper_tor_fw_helper_LDFLAGS = $(nat_pmp_ldflags) $(miniupnpc_ldflags)
 src_tools_tor_fw_helper_tor_fw_helper_LDADD = src/common/libor.a $(nat_pmp_ldadd) $(miniupnpc_ldadd) -lm @TOR_LIB_WS32@
-src_tools_tor_fw_helper_tor_fw_helper_CPPFLAGS = $(nat_pmp_cppflags) $(miniupnpc_cppflags)
+src_tools_tor_fw_helper_tor_fw_helper_CPPFLAGS = $(nat_pmp_cppflags) $(miniupnpc_cppflags) -I"$(top_srcdir)/src/ext"