Fix 'local' to not look in the source tree for the file.

Usually 'local' is used along with other rules such as 'no-implicit-rule' or
'dependency' which avoids this problem. It's possible to need to use 'local'
while relying on the default rules though for a file which is not in the source
tree nor generated in the kernel.

Sponsored by: Dell
Differential Revision: https://reviews.freebsd.org/D13125