build: add sysprof-diff tool

This is not installed currently. It's for testing now but we can start
installing it if/when people want to use it. We could also try to make
it part of sysprof-cli someday.

This tool prints the difference of two trees giving you a -/+ change of
the number of hits incurred on that frame-pointer.

src/meson.build

@@ -29,4 +29,5 @@ endif
 if get_option('tools')
   subdir('sysprof-agent')
   subdir('sysprof-cli')
+  subdir('sysprof-diff')
 endif