package/mfgtools: fix build issue related to __time64_t

package/mfgtools: fix build issue related to __time64_t

The tool fails to build on recent distros due to conflicting declaration
of __time64_t. Adding a check around the declaration to avoid
redefinition.

Patch not submitted upstream as the tool is not supported by NXP
anymore[1].

Fixes:
http://autobuild.buildroot.net/results/ca4498ad21a96ba2a38ca2467dadffdbb516355b/

[1] https://github.com/NXPmicro/mfgtools/pull/104

Signed-off-by: Gary Bisson <bisson.gary@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>