libcurl: add brotli optional dependency

libcurl: add brotli optional dependency

The brotli package has recently been added to Buildroot. Add brotli an
an optional dependency to libcurl to make the build consistent.

It turns out that libcurl configure script uses pkg-config to figure
out link libraries only when --with-brotli is explicitly set. So this
also fixes static build failure.

Fixes:
http://autobuild.buildroot.net/results/64b/64bc0dfe284206390ae0680b94c0876863a3c0f3/
http://autobuild.buildroot.net/results/233/23376d8653dea6361e42b0f17b6aaab3c14d99cf/
http://autobuild.buildroot.net/results/b19/b198db4b69e18e6d01ec95aae9c6096c1912dd9c/

Cc: Adrian Perez de Castro <aperez@igalia.com>
Cc: Matt Weber <matthew.weber@rockwellcollins.com>
Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Reviewed-by: Matt Weber <matthew.weber@rockwellcollins.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>