Update rpm.rb

Fix how buildroot is specified to better conform with the rpmbuild CLI. The old way was failing with `File not found` errors.

lib/fpm/package/rpm.rb

@@ -473,7 +473,7 @@ def output(output_path)
     args += ["--define", "dist .#{attributes[:rpm_dist]}"] if attributes[:rpm_dist]
 
     args += [
-      "--define", "buildroot #{build_path}/BUILD",
+      "--buildroot", "#{build_path}/BUILD",
       "--define", "_topdir #{build_path}",
       "--define", "_sourcedir #{build_path}",
       "--define", "_rpmdir #{build_path}/RPMS",