Brotli: link against libm.
Brotli: link against libm.

Change-Id: I4cf49089a679c8d2485799276277db8f0acae253
Signed-off-by: Piotr Sikora <ba6f3a1a6d7e1ecae91b31e93969cc392fbd0fdd@google.com>

google/ngx_brotli,google/ngx_brotli