[FS#1003] Flex does not build with GCC 7.2

[LEDE Bugs]

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FS#1003 - Flex does not build with GCC 7.2
User who did this - fseek (fseek)

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The commit [[https://github.com/westes/flex/commit/24fd0551333e7eded87b64dd36062da3df2f6380|24fd055]] is also necessary to fix the segfault.

Here is the patch I'm successfully using at the moment (place it in tools/flex/patches)
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More information can be found at the following URL:
https://bugs.lede-project.org/index.php?do=details&task_id=1003#comment3478