Bazel: link ":brotli_enc" with -lm.

While this isn't strictly necessary with recent versions of Bazel
(which unconditionally add -lm to linkopts), building Brotli with
older versions of Bazel requires -lm to be added explicitly.

Signed-off-by: Piotr Sikora <piotrsikora@google.com>
diff --git a/BUILD b/BUILD
index 1c49ceb..31b0b3e 100644
--- a/BUILD
+++ b/BUILD
@@ -86,6 +86,7 @@
     srcs = [":enc_sources"],
     hdrs = [":enc_headers"],
     copts = STRICT_C_OPTIONS,
+    linkopts = ["-lm"],
     deps = [":brotli_common"],
 )