schema/gen.go: Set -modtime=0 in 'esc' invocation

To give reproducible builds regardless of when you check the JSON
files out of Git.

Signed-off-by: W. Trevor King <[email protected]>

Author: wking

schema/gen.go

@@ -18,4 +18,4 @@ package schema
 // using esc (https://github.com/mjibson/esc).
 
 // This should generally be invoked with `make schema-fs`
-//go:generate esc -private -pkg=schema -ignore=.*go .
+//go:generate esc -private -modtime=0 -pkg=schema -ignore=.*go .