mod_brotli: Update makefile to cope with the pkg-config layout change

in https://github.com/google/brotli/commit/fe9f9a9

There's a typo in the upstream commit [1] (s/brotlicommon/libbrotlicommon)
that currently produces an unusable libbrotlienc.pc file, but hopefully
this is going to be fixed there.

[1] https://github.com/google/brotli/commit/fe9f9a9#diff-af3b638bc2a3e6c650974192a53c7291R409


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1779111 13f79535-47bb-0310-9956-ffa450edef68