package/squid: fix typo

Fixed typo added by
https://git.buildroot.net/buildroot/commit/package/squid?id=d2f7d0d72cd7e00ffbe869011d200f0a4a53e7a5

Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
(cherry picked from commit 7c5526c79cd74592cff55f8344216ad0a968119a)
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

package/squid/squid.mk

@@ -21,7 +21,7 @@ SQUID_CONF_ENV = \
 	ac_cv_func___va_copy=yes \
 	ac_cv_func_strnstr=no \
 	ac_cv_have_squid=yes \
-	BUILXCXX="$(HOSTCXX)" \
+	BUILDCXX="$(HOSTCXX)" \
 	BUILDCXXFLAGS="$(HOST_CXXFLAGS)"
 SQUID_CONF_OPTS = \
 	--enable-async-io=8 \