mod_brotli: Unbreak building other filter modules without libbrotlienc.

Don't add -lbrotlienc to the MOD_LDFLAGS unconditionally.  And, when
adding it, use MOD_BROTLI_LDADD to avoid linking all filter modules
against this library.


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1761824 13f79535-47bb-0310-9956-ffa450edef68