package/squid: fix build with host gcc 10

Pass -std=c++17 to fix the following build failure with host gcc 10
raised since bump to version 6.6 in commit
c13199c932:

/usr/bin/g++ -O2 -I/home/buildroot/instance-0/output-1/host/include -o cf_gen ./cf_gen.cc -I. -I../include/ -I../src
./cf_gen.cc: In function 'int main(int, char**)':
./cf_gen.cc:268:63: error: forming reference to void
  268 |                     auto &newEntry = entries.emplace_back(name);
      |                                                               ^

Fixes:
 - http://autobuild.buildroot.org/results/613fee008c77f8dbbe04df9a4ce4347e43de9ef9
 - https://bugs.buildroot.org/show_bug.cgi?id=15997

Reported-by: Roland Franke
Suggested-by: Peter Seiderer <ps.report@gmx.net>
Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Arnout Vandecappelle <arnout@mind.be>
(cherry picked from commit 9347905b95)
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

package/squid/squid.mk

@@ -21,7 +21,7 @@ SQUID_CONF_ENV = \
 	ac_cv_func_strnstr=no \
 	ac_cv_have_squid=yes \
 	BUILDCXX="$(HOSTCXX)" \
-	BUILDCXXFLAGS="$(HOST_CXXFLAGS)"
+	BUILDCXXFLAGS="$(HOST_CXXFLAGS) -std=c++17"
 SQUID_CONF_OPTS = \
 	--enable-async-io=8 \
 	--enable-linux-netfilter \