mysql: boost is only build time

According to the mysql release notes, Boost.Geometry is required to build MySQL (see https://dev.mysql.com/doc/relnotes/mysql/5.7/en/news-5-7-5.html).
However, Boost.Geometry is a header-only library, and only need to actually *build* MySQL...it is not needed as a runtime dependency.
This patch just changes the dependency on boost for MySQL to be build-time only.

See conversation at Homebrew/legacy-homebrew@df384ee for details and discussion that lead to this PR

Closes Homebrew/legacy-homebrew#47011.

Signed-off-by: Dominyk Tiller <dominyktiller@gmail.com>

Formula/mysql.rb

@@ -27,7 +27,7 @@ class Mysql < Formula
   depends_on "cmake" => :build
   depends_on "pidof" unless MacOS.version >= :mountain_lion
   depends_on "openssl"
-  depends_on "boost"
+  depends_on "boost" => :build
 
   conflicts_with "mysql-cluster", "mariadb", "percona-server",
     :because => "mysql, mariadb, and percona install the same binaries."