net/freeswitch: Fix build and disable -Werror to stop collecting -Wno-* flags

sip_pref_util.c:328:12: error: adding 'int' to a string does not append to the string [-Werror,-Wstring-plus-int]
    base = MATCH("audio") || MATCH("automata") || MATCH("application") ||
           ^~~~~~~~~~~~~~
sip_pref_util.c:315:31: note: expanded from macro 'MATCH'
  (su_casenmatch(param + 1, s + 1, strlen(s) - 1) && \
                            ~~^~~

http://beefy12.nyi.freebsd.org/data/head-amd64-default/p501065_s347375/logs/errors/freeswitch-1.8.1_2.log

net/freeswitch/Makefile

@@ -39,10 +39,7 @@ CONFLICTS_BUILD=	xmlrpc-c-*
 GNU_CONFIGURE=		yes
 CONFIGURE_ARGS=	--with-pkgconfigdir=${PREFIX}/libdata/pkgconfig \
 		--enable-zrtp
-CFLAGS+=	-Wno-c11-extensions -Wno-deprecated-declarations \
-		-Wno-zero-length-array -Wno-incompatible-pointer-types \
-		-Wno-ignored-optimization-argument -Wno-unknown-warning-option \
-		-Wno-unused-function -Wno-enum-conversion -Wno-reserved-user-defined-literal
+CFLAGS+=	-Wno-error
 
 USERS=		${PORTNAME}
 GROUPS=		${USERS}